It's always a pleasure to work with perfectly flat wood. However, sometimes the piece you want to use is cupped. Here are some suggestions you might find useful for flattening the wood.
If the piece is thin, try moistening the concave (incurving) side then clamping it firmly until dry. Often, this is all it takes to flatten the wood to a point where it can be used. However, try to use the wood as soon as possible, since it will tend to revert to the way it was originally. I do this with wood for box and bowl bottoms, and once glued into place, the wood becomes quite stable. I would not use wood that has been cupped and flattened this way for a slab lid or other unsupported use. It might remain flat, but it also might not, and why take the chance?
If the cupped piece is thick enough so you have wood to spare, place it cupped side up so that it is stable and put it through the drum sander. Eventually the top surface will flatten out. When this is achieved, invert the piece and sand the other side until it is flat.