As for the photos, I don’t have any better ones. I was too annoyed that I bought a fan that was too thick. I should have bought the NF-A4x10 instead of the NF-A4x20, so I had to improvise.
The steps required to get to the point where you can remove the PSU are very well described here:
https://www.ifixit.com/Guide/Framework+Desktop+Power+Supply+Replacement/189719
After that, it’s just the PSU itself. Disassembly is quick. In the guide above, steps 36–37 show the screws that need to be removed. The screwdriver included with the computer is sufficient for this.
The actual fan replacement and connecting it to power was a bit of a makeshift solution in my case. However, this video shows that it is also quite easy to do: