Release the magazine. You may want to take your slide stop and wiggle it to make sure it comes out properly. The first thing you must do is drop the magazine. Pull the barrel out of the slide.
Now it is time to remove the connector. Learn more. That was the disassembly and reassembly of the Glock 21 slide. With your right hand holding the slide a quarter inch back, use your left hand to pull down the slide release lock on both sides.
Next, hold the round bearing down when you pull out the extractor, or it will fly in your face. Take a flathead screw driver and lift the spring a bit. Put on safety glasses.
Open the slide. You can push the lever up with your thumb while holding the slide back with your other hand. For safety purposes I recommend not having a round in the chamber while not actively using the pistol. Now we have the gun completely disassembled.