The slide stop is removed when the rear of the slide's stop notch (rear point) is aligned with the rear edge of the circular notch around the slide stop pin (rear point, again), or, as the factory words it, align the slide stop notch on the slide with the stud of the slide stop.
Sometimes it can take more effort to push out the end of the stud in one gun than another. The stud has a semicircular notch in it within which rests the end of the plunger for the guide rod, and you have to depress the plunger by starting the stud moving (and without much in the way of gaining leverage advantage by "getting a running start" at it, either, since the curved notch is short
).
Sometimes using the end of the orange chamber safety flag which comes with the gun can help get it started (using the tip to push on the right end of the stud).
When reinstalling the slide stop lever's stud into the frame, make sure the barrel is all the way rearward, or else you'll be pushing the steel stud against the front of the barrel's bottom lug ... and you'll get tired before the steel parts do.
One way is to hold the pistol horizontal and just push the barrel back with your finger, and another way is to tip the pistol slightly muzzle-up so gravity can work and drop the barrel back to clear the frame's stud hole.
When a magazine is fully loaded and being inserted into the grip's mag well with the slide forward in battery, it often takes a brisk push against the bottom of the mag to properly seat it so the mag catch is fully engaged. This is because the full mag load (the rounds) has to be depressed even further when the slide's pick-up rail is pushing down on the rounds in order to position the mag high enough for the mag catch to engage.
This is especially noticeable with new springs, and it's not uncommon among pistols.
