This is how I've done it in 2 21.5" iMacs, a straight HDD-SSD replacement. Some steps may be different if you are looking to get rid of the optical drive and put the hard drive in its place.
1. Clone the contents of the existing hard drive into the SSD, using a 2.5 external hard drive enclosure (can be had for cheap on Amazon - I personally use an Orico).
2. To check that it works, boot from the SSD while it's external (by holding Option on start-up and selecting it)
3. Put the SSD inside the 2.5-3.5 enclosure
4. Open-up the iMac, take out the existing hard drive, and replace it with the SSD, using the OWC sensor cable etc. The OWC video has quite detailed instructions, though it simplifies many of the steps. In particular, BE CAREFUL WITH THE DISPLAY CABLE - this has quite a few pins that can easily be damaged. I damaged the cable when I did this once, though thankfully you can buy a replacement for about £15 on eBay. Also, the screws that hold the screen in place can be a total nightmare to put back in because the iMac is full of magnets (normally used to hold the glass screen in place) that will draw the screws. Make sure you buy the OWC kit including the tools, as it will have the suction cups to remove the screen, as well as all the screwdrivers you'll ever need for Mac repairs.
5. Your computer should boot from the SSD now. Make sure once you are in OSX, you click on 'start up disk' and click on the new Macintosh partition. Otherwise, your start-up will be slower as your computer will always first look for the original HDD, before moving to your new hard drive.
6. You're done!
P.S: You may also want to get a 3.5 enclosure, so that you can recycle your iMac's previous HDD as an external.
Thanks for your reply. So do I just replace the stock HD or add a second SSD and somehow get the computer to boot from the second SSD?