Naturally, you have to remove the slide first. If you look at the metal ring (the slide nut) on the front of the slide you will see two notches. Now, the first thing you need to understand, is that when the nut is screwed on, the the metal in the notches is peened to keep the nut from loosening. The best way to remove the nut is to lock a piece of metal, with the same thickness of the notches in a vice, so that about 1/4" is protruding from the top of the vice. The one piece of metal has to be long enough to engage both notches. You then position the metal in the notches, and while applying downward pressure, you unscrew it. It will take a little muscle because of the peening. To reassemble it, you pretty much reverse the order of disassembly, remembering to peen the notches when you're done.
