Flatten Angled Cuts with Elbow Grease and a Sanding Pad
First, coat the back sides of a sheet or two of sandpaper with spray adhesive and mount them on a flat piece of hardwood plywood. Then, clamp the sandpaper-coated plywood to your bench, place the wood to be flattened on the sandpaper, and work it back and forth in the direction of the grain. Use even pressure and take care not to rock the workpiece. With careful sanding, you can level the workpiece as flat as the surface of the plywood.
—From the WOOD® magazine shop