I just stuff them with my glass beads about half way up, them finish filling them with poly fill. Some artist use glue to seal them however I haven't had any problem with just stuffing the limbs tightly with poly fill.
I fill with glass beads and then some polly fill. That helps the beads from sloshing around. Then take some felt or heavy cardboard and glue them over the hole in the limb. Just what I do, if the kit doesn't come with plugs. Hope this helps!