A sanding block isn't rocket science, but does come in very handy for a variety of sanding tasks. Go to a store and price a sanding block and you'll wonder why anyone ever paid money for one This project might take a whole six minutes of your life.
Step 1: A Block of Wood Cut to Size As Long As Sandpaper Will Fit It.
This is really easy stuff, but people will run out to buy sanding blocks that do absolutely nothing more than a block of wood will do . Just make sure that the block of wood isn't too big for a sheet of sand paper. I like to bevel the edges slightly just to keep sharp edges from digging into the wood I'm sanding.
Step 2: Add a Cusion.
The cusion just lets the sand paper flex a bit. You probably have something suitable sitting around. I like using foam packing material. Just cut to fit the block of wood then glue it to the wood block just enough to get it to stick.
Step 3: Add Clips.
Those cheapo binder clips work great and you can find them almost everywhere. Just screw them to the opposite side of the cusion and use a washer so they don't fall off.
Step 4: Put It Together.
Just clip your trimmed sand paper to the block and you're in business.