As in any pistol maintenance, MAKE SURE YOU FIREARM IS UNLOADED, THREE TIMES!!!
A small flathead screwdriver... First make sure the set screw is flush with the bottom of the sight... With the sight off and ready, standing by, use the flat part of the screw driver blade side and press the plunger into it's cavity, holding it down using the screwdriver from the slides left side... Then with your third hand, put the sight in from the slides right side... Just until it covers the plunger pretty good. Then do the "table cloth" trick by removing the screw driver very quickly. You should here a "click as the plunger impacts the bottom of ths sight. Verify that the plunger is still in place by peaking in the area with a good light. You should see it protruding straight up fom it's cavity... You're home free now! Verify the function of the plunger by pushing the firing pin block from the bottom of the slide up and down... It should move freely, under spring tension. Once satisfied the plunger is good to go, finsish the sight install. Lock the set screw pretty snug... and re-assemble the pistol. Then do a pencil test, a number 2 pencil, in the barrel, eraser first. Point the pistol in a safe direction and pull the trigger... the pencil should launch, if you've done the job correctly.
The P95 is typical of the other 'P' series pistols except for the P345. You loosen the set screw slightly, do not remove it. Drift the sight out from left to right as if you were shooting it. When replacing, re-install from right to left. Get the sight started, then use a small screwdriver to hold the plunger down while drifting the sight until you can no longer keep the screwdriver in place on the plunger. Drift the sight the rest of the way in. The set screw hole passes over the plunger so be sure the set screw is in place and even with the bottom surface of the sight. If you don't you'll break or bend over the plunger and the gun won't work.