Flattening bumpy wood involves several steps. First, you need to identify the high spots on the surface of the wood. This can be done by running your hand or a straightedge across the board and feeling for any raised areas. Once you have identified the problem areas, use a plane or a belt sander to carefully remove material from those spots until they are level with the rest of the wood. It's important to make sure you remove material evenly and gradually, so as not to create any new low spots. Continue checking the flatness regularly as you work, making adjustments as needed until the entire surface is smooth and even.