Bill,..if you are struggling with this at all..please share some more detail and we’ll help.
Many crystals for vintage watches are simply pressed in, with a good snug friction fit.
Some you can put in by hand,..some go better with a press.
This of course applies mostly to acrylic / plastic crystals.
Glass is a whole other “sport” if you will,. but it can be done with a bit of knowledge
Best of luck,….
Randy