Free: Contests & Raffles.
Only done a few myself but heat definitely speeds up the process and gets it cleaner. It's summer time so you could get away with just leaving it outside in the heat. But as it gets cooler I used a fish tank heater.
Cold water is how I do it. Works good, I put the skull in a plastic box and fill it up to its eye sockets with water, then slip a garbage bag over it to keep the moisture in. If its a deer or elk I also put another black garbage bag over the antlers. I put it out back away from the house under my pine tree and then its ready in 2-4 months.