I did this very thing last week. That article linked above goes waaaaay too far in taking the iMac apart!
Steps:
- Remove RAM.
- Remove screen.
- Remove IR thing behind the Apple logo.
- Disconnect everything from the logic board.
- Remove logic board.
- Remove optical drive.
- Remove existing pressure wall.
- Add new pressure wall with SSD attached.
- Replace power cable with new power cable.
- Connect everything back to the logic board.
- Replace logic board, IR thing, screen, RAM.
Done!
Still, I'd never do it again. You need surgeon-like dexterity to get all the little connectors out of the logic board and the logic board out then back in the machine without damaging anything! I'd happily pay an Apple Service Provider to do it for me.