step 1: remove all visible large particles such as roots and dried dead leaves from DRY hydroton.

step 2: Place all hydroton in large container preferably with holes at the bottom or the sides (such as a bucket or net pots)

step 3: Use garden hose to rinse out most of the other particles (rinse thoroughly)

step 4: Use near boiling water or the hottest water you can get and pour over the rocks. The water obviously should be free of particles itself.

as a side note, you can omit step 4 and rinse with only cold water and rubbing alcohol at step 3. This will kill any organisms. Be sure to rinse thoroughly of course and let rocks dry before using.