Wanted to add an SSD drive for performance, discovered the dual drive product from iFixIt and thought, "perfect!"
Familiarize yourself with the instructions and diagrams by reading through them a few times before you dive in. Be sure to heed their advice about how gently you need to lift things etc. There are a number of connectors & wires that don't have much slack, so you really have to be careful when lifting the screen & logic board out.
Make sure that you are working on a surface where you can rotate the iMac 180 degrees: some of the tutorial has it oriented facing up, and some has it facing down. Also, note that the teardown took about 45 minutes, but the rebuild took about 15. It goes much faster once you've undone everything. Be sure to keep all your wires clear when you reinstall the optical drive and logic board! You don't want to get halfway into the rebuild only to find that the fan sensor wire is stuck under the logic board!