Very helpful guide, easy to follow. Took me around an hour and a half to install an SSD.
For the record, instead of using them independently I configured them as a Fusion Drive. Works like a charm, more info here:
Having read this comment, I initially skipped the PSU removal steps. While removing the logic board was a bit difficult, putting it back in place seemed impossible until I figured one of the plastic walls (step 37, although it was the one on the other side) was getting in the way: some pieces have a diagonal cut from top to bottom and they have to fit together, and the diagonal shape forces one to be placed before the other (one on bottom, then the other on top, so to speak). I had to remove the PSU to be able to remove the plastic walls, get the logic board in place properly, and then put the plastic walls back "on top" of a piece attached to the PCB.
So while I guess it could be possible to skip the PSU removal steps, I found it easier to put everything back into place by removing it. YMMV, just keep it in mind, and you can always do as I did, skip the steps and do them afterwards if needed.
As another user already posted, cables can be in the reverse order as depicted on the guide. Pay attention before disconnecting them.