Elizabeth, where do you want the glitter to end up? You can prepare you mold with mold release first, then mix up some glitter with a bit of resin and paint onto the inside of your mold.
Pearl powder works great dusted on the inside of a mold with resin poured in it. You can see what I’m talking about here: https://resinobsession.com/resin-tutorials/using-pearl-ex-resin/
If your’e wanting glitter behind your casting, and the back is the last layer you’re casting, wait until the resin starts to cure, then sprinkle glitter on the back. I do it in this video:
https://youtu.be/DZPc4y_j7ng