Find the center of the twist.. or just use the center point of the stick. Using a 2ft level, or a torpedo level, level this point by shimming either end. Mark where you placed the level so you know where to reference in case the timber gets bumped. Then go the end grain and draw a level line to remove as little wood as possible. If you need to surface more than one face layout a plumb face as well. Then use a chalk line to connect the points. Start planing... you can use the level to check your progress. Plumb and level will always remain constant and square to each other provided that gravity doesn't change. Start by planing across the grain to remove the bulk. Following these steps will take the twist out and straighten it. Hope this helps.