Make sure all updates are installed before you start. In particular the firmware update. This model can use internet recovery to download OSX after you put in the SSD.
You need a screwdriver and keep track of which screw goes where. Take a piece of paper, draw circles on it, and label the circles based on where you got the screw from. There should only be two types of screws (outside screws and hard drive screws) but this is good practice.
After everything is put back together, you turn on the mac and hold down ⌘ + R until you get to the recovery screen. You can then use the reinstall OSX option and follow the instructions. It will be a 5 GB download and some installation. On fast internet it should take about an hour.