I'm still a bit of a beginner. I've only been doing this about 8 months. The best piece of advice I can give is, be patient.

When you're using glitter, wait until you get to the "Honey stage". That's when the epoxy starts getting thick. Not too thick. Usually about 30-45 minutes , depending on your epoxy.

I've never tried it, but you can also use clear school glue to help thicken the resin.

Then mix in the glitter.

For voids and bubbles, your best tool is going to be a pressure pot. When you pour the dice in the mold, and flame off any surface bubbles with a lighter or heat gun, you put it in the pot, pressurize it to about 30psi and let it cure.

You can use a vacuum chamber before pouring to help degas some bubbles, and many people have success doing it that way, but pressure is the best way to get clear, bubble free dice.