Check out the Heartfelt website for instructions.

I add DISTILLED water until they are about half soaked. Check the beads for several days. If they seem to have dried out quite a bit, add back to the same level you previously wet them. You're good to go for weeks. Just keep an eye on them and never add anything except distilled water. Presto! 65% humidity.

The beads probably should be replaced after a few years, though some people think they will last indefinitely.