I like to use a good detergent (I prefer Dawn dish soap) and warm water. Squeeze the mold and pull it apart to get any resin crumbs out. Assuming all the resin has cured, any resin fragments should slide out easily.
Rinse well, then slap it onto a towel to ‘beat out’ the remaining water. If you have hard water, it can leave spots after it dries. You could also do a final rinse in distilled water or even warm it with a heat gun to evaporate any remaining moisture.