It's more a matter of dexterity than anything else. I believe the instructions recommend using a wooden toothpick to release two spring clamps, which allows the OEM focus screen to "hinge" down and out of place. You then simply reinsert the new focus screen into place using the same (but reversed) "hinge" method.
As expected, the most frequently mentioned problem is finger prints or debris on the new focus screen, so it will require a little care and/or maybe a couple of tries to get it installed without prints, smudges, or debris. Most posts that I've seen regarding installation were trouble/problem-free.