The way I was taught to sand drum shells is; sand-paper held in your hand, (no sanding blocks or machines) and going with the grain, you make the longest strokes around the shell that you can. The key is l-o-n-g strokes. Don't sand back and forth in one little area. Make the strokes as long as your arm will reach around the shell. It'll keep the surface even/round with no flat spots.
You don't need -any- tools, just the meat-hook at the end of your arm. Your hand will shape the paper to the curvature of the shell better than any flat sanding block or the flat base of a sanding machine ever will.
The rest is elbow-grease.
John