You melt them back down in a pot on a furnace, like they were made in the first place. Probably wearing a big giant filtration mask and gloves to be safe, but mildly overcautious.
You tube How-to Review of one brand's lead furnace for educational, not marketing uses. This video will give you an idea of how it's done, but this is in no way an endorsement of that brand, person, etc etc.
ETA: Here's a probably better video as the guy actual melts lead...
Another video.