Excellent guide. Thank you!
I got through everything, turned on the iMac, got the start up gong, and ... nothing. Screen stayed black and the fans started blowing like crazy, indicating failure of Apple Hardware Test. All my attempts to remedy (zap PRAM, reset SMC) failed. Uh oh.
Opened the machine back up, took out the LCD, and re-seated all four connections from display panel to logic board. Crossed my fingers and put it back together. This time, the displayed turned on, the Apple logo appeared, system booted up, and the new SSD appeared on the desktop. Yay!
The connectors from the LCD are delicate. It's tricky to seat them properly without damaging them, but don't close it up without a positive click on each one.
The upgrade has been fantastic so far. SanDisk 120GB SSD. Dramatic speed improvement. Well worth all the surgery on the machine.