Clamp it to a table with two blocks on either end with the clamp in the middle and the bow out facing up. Adjust the clamp until the wood is straight, then tighten it slightly every couple of hours until its bowed in the opposite direction. Keep it in the clamp for a couple of days, then remove. If its not straight place it back in the clamp setup and tighten more. Be very careful not to overtighten or it will crack. If the clamp becomes loose then tighten some more (it will become loose as the wood bends into shape). The adjustment must be done slowly. We have used this method to straighten necks from low-end kits to a Warmoth neck.