These are my favorite sanding blocks that I use constantly. I keep a 100 grit and a 60 grit block handy at all times (aluminum oxide paper). They may not be much to look at, and making them may seem like a no-brainer. But I wasn't aware of the fine points of sanding block making until someone showed me years ago, so here goes for those who are still suffering with those awful hardware store sanding blocks...
The core of the block is simply hard and stable wood, cut to approximately 3/4" x 3 3/8" x 11". The best material, shown here, is baltic birch plywood. Cabinet-grade plywood also works well. You can also use high-density particle board or MDF with good results, but don't expect it to be as stable or as long-lasting as the birch ply. Solid wood should be avoided. These blocks are frequently used for leveling surfaces or sanding straight lines, so straightness and flatness are important. For some jobs you may want to have a board with one cork face for cushioning (which also helps the sandpaper to last a little longer), but mostly I prefer hard blocks for airplane building.
Line up the edge of the block with the edge of the sandpaper.
Carefully roll the block over 90°, then lift up the sandpaper's edge to form a soft bend at the first corner.
Fold the first bend over 180° and smash it to a sharp bend with the block.
Repeat this procedure for the rest of the bends...
...until the short edge overlaps. If the final edge of the sandpaper sticks out too far, the block needs to be wider or thicker. You shouldn't ever need to trim the sandpaper when the block is sized properly.
Complete the sanding block with 5 staples on the overlapped edge only. You now have a versatile sanding block with 3 useful surfaces.
How much sandpaper have YOU had in storage that just curls up and dies on you? Storing sandpaper in hanging files is the high-tone way to go, and here the files are labelled by grit. This handy file box keeps the dust and spiders out of your sandpaper, too. For about a $10 investment, you'll always have flat and clean paper when you need it.