I don't have a P9, but I do have a K40, K9, and MK9. Disassembling them is quite easy once you learn the trick. First, remove the magazine, lock the slide open, and check that the chamber is empty (both with your eyes and your finger). Check the chamber again. Second, pick up the gun with your right hand. Place the palm of your left hand over the front of slide and put your left thumb through the trigger guard. Using the left hand only, you can now retract the slide to the takedown marks and hold it there. While keeping the slide retracted, rotate the gun so that you can see the right side of the gun. As VictorLouis suggested, use the back of a plastic handled screwdriver to the slide stop pin until it no longer protrudes. Then pull the slide stop out the other side.
The key is learning to both hold the gun and retract the slide with one hand. Until you figure that out, you end up needing three hands (one to hold the gun, a second to retract the slide, and a third to tap out the slide stop).
M1911