Start with the slide cut aligned with the frame opening and with the slide stop in this position. Gently press inwards and upwards. The trick here is to push the slide stop so that the plunger does not get into the slide stop slot, but below it, so that it gets to the flat part of the flat slide stop face.
Here you see the plunger as it starts climbing to the flat slide stop face. Keep pushing inwards and upwards.
Here you can see that the slide stop has entered the frame and the slide cut, and is almost fully into its final position.
I've came up with the following two video clips, trying to better show you, how to insert the slide stop, without scratching your pistol.