DETOUR WITH A HAND PLANE
If the board you are trying to flatten is wider than your jointer or you don’t have a jointer, you’ll have to employ another method for flattening it. Start by placing your board on a flat surface, like the top of a table saw. Press down on all the corners. The amount of wobbling or rocking translates to the amount of material you’ll have to remove until the board sits flat.
Mark the high spots with a pencil and take a few passes over them with a plane. A jack plane, usually about 14” long, is my choice for this operation. Check the board again on the flat surface. If it doesn’t move, you’re done. If it does, keep removing material in small increments until the board is flat.