two retaining pins come out and the trigger block, spring and guide are all freed up. If you need to take the piston out it's easiest to remove the barrel and cocking mech from the reciever so it can slide out the back(watch out for sharp edges in the cocking slots they may be sharp and damage the piston, or you).
The tricky part is durring reassembly when you go to put the retaining pins back in and the safety catch spring that gets it tension from the pin closest to the reciever end gets in the way. With all of my Dianas I have found that the pin closest to the barrel end of the gun doesn't give much trouble and can be wiggled into place. Then the rear pin can be pushed through easily while you hold the safety button in so the bend in the spring is in line with the hole in the reciever, and further move it out of the pins path with a small flat blade screw driver and push the pin through. I've done it this way quite a few times and never had any problems. Good luck, and have fun. Mulby