We are going to test your dexterity in this step :)

Prepare the following hardware:

2x M4x10 screws (reused from your Original Prusa).

2x psu_upper_mount.

Place the M4x10 screw in the internal hole of the psu_upper_mount.

If you struggle with this, you can insert a finger in the biggest hole (check the 3rd image). doing this makes it easier for you to hold and guide the screw into the hole.

Insert the other M4x10 in the remaining psu_upper_mount.

Now try not to lose the screws until we are bolting them on the PSU :D