Purchase a bag of clear plastic craft beads. Most craft stores keep these in stock and they are relatively inexpensive.
Acquire a metal or aluminum ice cube tray. Several online retailers sell metal ice cube trays. If you can't find one, see the resources to learn how to make a mold yourself from aluminum foil.
Fill each mold to the top with the plastic craft beads. Preheat your oven to 375 F.
Bake the beads for 15 to 20 minutes to melt them down. Once they are finished, let them cool for about an hour, and then remove the "ice cubes" from the mold.