Marc St. Louis wrote a chapter on heat treating/tempering in The Traditional Bowyers Bible Vol.4. If your going to be building bows you'll want a copy of the book anyhow.
The procedure is pretty simple, clamp your bow to a caul with a little bit of reflex and heat the limbs slowly till the wood begins to turn a toasted brown. But as Diamondback59 said you may want to make sure the bow is dry first or it could check badly.