Check out You Tube videos and the iFixit Guide.
Here is the HDD replacement guide. It will not be exactly what you will be doing, as you are installing an SSD, but the steps are all the same until the very end:
This guide will help you replace/upgrade the hard drive. This guide also includes steps to upgrade your iMac's hard drive with a SSD. It describes...
www.ifixit.com
Look over the guide, and watch some YouTube videos, as the guide alone might leave you with some questions.
For example, on step 7, that could be confusing just by reading the guide. With the iMac laying on its back, you GENTLY pull the tab upward toward the sky to unlock the locking bracket, and then GENTLY pull it toward the top side of the iMac to pull the connector out.
If you read the iFixit guide, and watch some videos and still feel uncomfortable doing the swap, consider this advice:
I didn’t feel comfortable doing this upgrade to my wife’s identical iMac some years ago so farmed out the task to a local Apple authorized shop. They did the swap for $110 labor. With a SSD I provided. Felt it was worth the cost at the time - done in Nov 2013.
If you don't feel comfortable doing the swap, and paying someone to do it just isn't an option, you could try an external boot drive, such as Thunderbolt. TB options are sometimes hard to find and might be expensive.
Even using a SSD over USB2 or FW800 might be worth it over using an internal HDD. The sequential speeds would not be as high on USB2 or FW800, but the random speeds will most likely be much better than using a HDD. This would give you that "instant" response from the OS.
You could also use a SSD over USB3 via a TB dock with USB3 ports, which is easier to fine than a TB1 or TB2 enclosure/SSD.
It probably wouldn't be worth the investment, but you could get a TB3 NVMe SSD, which would be the fastest non-RAID drive possible for your iMac (900MBps). To get this working, you will also need a TB3 dock, and the Apple TB bidirectional adapter, so it is expensive. I have this set up for my Late 2012 iMac.
All that said, it isn't that complex of an install. Of course things can go wrong, and you can break stuff, but for Macs this install isn't that big of a deal.
The iMac would feel like a new computer with a SSD installed, and it is totally worth it imo.
If you choose to go forward with the install, read further:
In a little bit, I will post some general steps of what I think you could do.
Start with prep work:
Back up your drive - While not required, it is just general good practice to have a backup of your data. A bootable back is better, but TM, or just a manual back up of important stuff is fine.
High Sierra - Update/Fresh Install the OS on the internal HDD to High Sierra and complete all the updates.
Firmware - Check the firmware after all the updates - Apple symbol > About this Mac > System Report > Hardware Overview > Boot Rom Version. It should say "87.0.0.0.0" for the Mid 2011 iMacs.
iFixit Guide - Have this handy on another device.
Buy what you need for the install:
SSD - Get an internal SATAIII SSD. Many like the Samsung EVO 860. I have used them and they were great, but so was every other SSD that I have used, so don't feel the need to pay more if the cost is an issue. Get a size big enough for your needs, leaving at least 10% free space after everything you plan on storing on it.
3.5" to 2.5" SATA caddy - This is not required, but are super cheap and keeps things clean. I have used a few different ones in the past, such as some IcyDock ones. Don't spend a lot on a caddy, if it cost more than $10, it probably isn't worth it for this install.
Some advice, try to fine ones
without a build-in SATA PCB, as you do not need an exact match up for the SATA ports due to being cable connected. This is an example of one that is NOT recommend for your iMac:
Fenlink 2.5" to 3.5" Internal SSD Hard Drive SATA Drive Converter (Silver)
www.amazon.com
If you might consider a dual SSD in the future, I purchased this one for a dual SSD set up in a Mid 2011 iMac:
Shop CORSAIR Dual SATA Drive Enclosure for 2.5" Solid-State Drives Black at Best Buy. Find low everyday prices and buy online for delivery or in-store pick-up. Price Match Guarantee.
www.bestbuy.com
You can use the dual caddy for a single drive as well. It is really cheap @$6, so just a good option for both single or dual SSDs.
If you want to skip the caddy all together, you can just use double-face tape, and stick in the HDD bay.
USB/SATA Adapter Cable or Enclosure - Completely optional, but it is helpful for externally formatting and testing the new SSD prior to installing it internally. They are nice to keep around for troubleshooting, back ups, or even to have an external boot drive. I like this one:
Product Description:Take advantage of the lightening speed of USB 3.0 to quickly transfer files up to 5 Gbps! Connect any 2.5'' Serial ATA (SATA) / SSD or SATA II Hard Drive to your computer through an available USB 3.0 port on your desktop or laptop. Reverse compatible with USB 2.0 and 1.1, this...
www.amazon.com
There are also cables for both 2.5" and 3.5" drives as well if you want to have one for your HDD.
SATA Thermal Sensor Adapter - It prevents your fan speeds to go full blast after replacing the HDD. It is optional because you can use a SW solution (MacsFanControl) to control the fan speeds, use other sensors that your splice in yourself, or just short the sensor wires to make the OS think that the temps are low.
Personally, I recommend just getting the SATA Sensor adapter, as it is an easy, no-hassle way of setting up your new SSD, and will give accurate temperatures, and let the OS decide what the fan speed should be. OWC and iFixit make them.
Here is the OWC version, although I have seen them cheaper on eBay sometimes:
OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade. OWC DIY Kit for all Apple iMac 2011 Models for replacing the main hard drive. Includes all cables necessary for installation. Hard drive and tools sold separately. Eliminate fan noise and maintain correct system fan control when...
www.amazon.com
Install:
1. SSD Setup (optional) - After doing the prep work, and purchasing everything, connect the SSD to your iMac via USB (or your way of choice), format the SSD using Disk Utility. Use Mac OS Extended (Journaled) and GUID Partition Map as the options. It should look like this:
1.1 Install High Sierra on the SSD - When setting up the OS on the SSD, you can migrate your data from your HDD if you choose using Apple Migration Assistant. You can also just clone the HDD to the new SSD using CCC or something similar. If you clone, boot into the SSD externally at least once to make sure cloned okay.
1.2 Disconnect everything - Once done doing the optional install of High Sierra and/or migration data, cloning, etc. you can shutdown, disconnect all cables, usb, TB, ethernet, power, etc.
Step 1 is optional, but it lets you know the drive works before installing it. You can do all of that after the drive is installed as well.
2. Get all your tools, parts, guide, and it is helpful to have someone to help you.
3. Follow the steps of the iFixit Guide with maybe some exceptions. I think the guide has your remove the RAM, this isn't necessary for the HDD swap, but is for removing the logic board which you are not doing.
The following is my steps to be used with iFixit Steps to help clarify things while giving optional additional things to do.
3.1 Display glass - You have done this, just be careful not to pick it up by the suction cups, as they can release easily. Also, do not touch the clear side of the glass!
3.2 Display screws - All eight screws for the display are the same T10 screws, so no need keeping track which hole they go in. It is easy to lose some of the screws down the gaps, but the magnets that are used to attach the display glass are helpful to catch them before the drop.
Be careful not to touch the display screen with your tools and hands!
3.3 Display Lift - Having hooks to lift the screen after the display bolts are out is really helpful. I made some with a clothes hanger, but paperclips would be easier to use as long as it would be strong enough to lift the display.
DO NOT lift the display more than a few inches, as there are cables that need to be disconnected first. This is were a friend to help you is nice. I have done this many times without help, so having someone with you isn't required.
If you need to set the display down for whatever reason, a tip is to prop it open with something like the rubber handle of a driver.
There are four connectors that need to be disconnected. These can sometimes be a pain, especially the first time and/or if you do not have someone to hold the display for you while disconnecting the connectors. They all disconnect in the direction of the top of the iMac. There is also a shock risk, pay attention to the iFixit guide where this is.
I already explained above about the cable ribbon with the pull tab. The pull tab is used to first unlock the bracket holding the connector by pulling it upward the display (perpendicular to the logic board), then toward the top of the iMac to disconnect the ribbon from the logic board.
Make sure all four are disconnected, pull the display out, and put in a safe place. You might also want to clean the dust off the back of the display as well, just be careful not to get it all over the front of the display.
3.4 HDD Removal - The HDD bracket is held in place by two T10 screws. Remove the SATA cables, and pull out the HDD.
3.5 Clean up (optional) - Your iMac has probably had 10 years of dust collecting inside. This is a great time to use some compressed air and blow out the inside. One the obvious stuff is gone, concentrate on areas that air is pushed or pulled, like fans and vents. You might want to do this outside, because dust will be flying. Maybe wear a mask too.
3.6 SSD Install - If you have the optional caddy, put the bracket and mounting pegs from the HDD to the caddy. You will need a T8 driver for this. Just make sure the bracket is on the correct side of the caddy, and the orientation is the same.
If you are using the optional SATA temp sensor cable adapter, you can choose where you want to mount the sensor using the stick side. I have mounted it on both a caddy and also directly to iMac, near where the vent hole is.
Also, if using the sensor adapter, you will have to tuck the SATA cable somewhere to get it to fit. I tuck it toward the GPU heat sink a little to get it to fit.
If using double-faced tape, just mount it where the HDD went, but over a little giving you room for the SATA Cable Adapter if you are using one.
Make sure the SATA cables are seated!
3.7 Put everything back together - Reverse what your did to take everything apart.
Two T10 Screws to mount the HDD bracket to the iMac.
Gently slide the display in place at the bottom of it and the iMac. Be careful not to touch the display screen with your tools and hands!
Four cable connectors, most just push right back in, but the ribbon cable, gently push in place, and move the bracket tab toward the bottom of the iMac to lock it in place.
Gently lower the display in place.
Eight T10 screws for the display (or just the top two and bottom two if you want to wait to make sure it works). Putting these screws back in is the hardest part for me. I worry a little about them dropping into the gaps, and while the magnets usually prevent that, the magnets makes it hard to get the screw into the holes.
The last few times, I actually used a small needle nose pliers to hold the screws in one hand, and tighten with the T10 driver with the other.
3.8 Replace the display glass - This is optional if you want to test everything first.
4 Testing and TRIM - Connect the power cable, keyboard and mouse, and boot up. If you did not already install the OS on the SSD, you will need to connect the HDD or some other back up to the iMac externally to boot from.
4.1 Fans - If you choose not to use a sensor, you probably hear the fans on full blast. You can fix this later with a SW solution. If you used a sensor, and the fans are still going nuts, let us know for some trouble shooting.
4.2 Format and HS Install - If you already installed the OS on the SSD, skip this step. Format the SSD in Disk Utilities if you have not already in a previous step. Install High Sierra on the SSD, and migrate your data.
4.3 Enable Trim - This requires using Terminal, but is super easy and is basically copy and paste. Open Terminal and type or paste the following then press enter:
You can check to make sure that TRIM is enabled by Apple Symbol > About This Mac > System Report > Hardware > SATA/SATA Express > Your SSD > TRIM Support.... Or look at this:
5. Enjoy!
It may seem like a lot of steps, but it isn't complicated. The whole task, from step 1 down to step 5 could be done in less than 20 minutes with someone that is proficient with Macs. Of course, if you are migrating a lot of data, that would add some time, especially with USB2.
Depending on your skill level, doing this for the first time might take 45 minute to 1.5 hours.
Let me know if something isn't clear, or if I made a mistake.
Also, let us know if you end up doing the install.