You can buy sanding blocks at the hardware store, but I’ve always made my own. All it requires is a small scrap of hardwood and a couple of passes with a saw. If the block is carefully cut and the sandpaper is crisply folded, the paper will stay snugly in place without gluing or tacking it.
Cut a block 1-1/4 in. by 3-1/2 in. I use this width and height because the block feels comfortable in my hand. Now cut the block in. long, the same length as half a standard sheet of sandpaper. A 5-1/2-in. deep kerf along the center of one of the long edges completes the block.
As shown in the drawing, there are six folds to be made in the sandpaper. Begin by inserting one edge of a half-sheet in the kerf. Fold the paper around the surface of the block until you reach the sixth fold. Gauge this crease by removing the first edge of the paper from the kerf, and inserting the second edge.
Once the sandpaper is all folded, hold the edges that will sit in the kerf together so that the sandpaper forms a sleeve, and feed it onto the block from one end.
M. Felix Marti, Monroe, OR