I've only owned Colts in the past so this side stop issue is new to me. On Colts, its just pops right in. I used the little gismo that snaps onto the recoil guide rod. Then, with the slide almost in the proper position, I used my thumb nail to depress the plunger until the slide stop engaged it. Then, with the plunger in its groove and the slide stop resting against the slide, I just had to move the slide a little to drive the slide stop home. Such an ordeal.