Remove any stain or finish on the board. Pour denatured alcohol onto a clean rag and wipe down the board. Start at one side and work toward the other, wiping down the board until all of the finish has been removed from the surface of all sides.
Lay the board outside on a warm sunny day. Place it so the twisted ends are touching the ground and the board is warping up. The middle of the board may not even be touching the ground in this situation.
Dampen the board with a garden hose. Spray water on the ends of the board until they are moist. Dampen a few old cloths and set them under the board at each end. Set cement blocks on top of the board along the entire length and leave it out in the sun for a few days.
Remove the cement blocks from the top of the board and take the damp cloths away also. Inspect the board and verify it has straightened out. Place C-clamps every 12 inches down the entire board and tighten them down in place throughout the entire length of the board. Use pads or small blocks of wood between the clamps and the board to protect the straightened board from the clamps.
Remove the C-clamps every day and move them to a slightly different position to avoid splitting. Continue this process for two weeks until the boards are completely straight.