[Drakker]

You have to hold the beaver tail safety while sliding the grip in. Don't let it go until you have secured both top screws on the grip. If you let go of it while screwing the top screws, it will slide slightly out of alignment and do exactly what you described.

If the grip is sliding off because you can't secure the stripped screws in the grip, get new screws, they are really cheap. If the screws holes in the metal frame are stripped, you can try slightly larger screws from the hardware store, but good luck finding screws of exactly the right length, you will most likely have to find longer screws, cut them to the desired length and rework the end with a small file. Although, if you have never done this before, you might be better off just sending it to a gun doc.