To flatten a board in a planer you would need to make a sled for it, then support the warped wood with shims or something like that. Both the planer and jointer are not recommended for boards shorter then 12 inches for safety reasons. There is a lot to take into consideration. If I have a board that has warped or twisted, my preferred method for flattening it is winding sticks and a sharp hand plane. It is accurate and quick, then you can run it through the planer to achieve the thickness you want.