1. Paint the stops of 20 coasters in assorted colors. Paint a second or third coat, as needed, to fully cover the cork.
2. Glue a plain coaster to the back of each painted coaster. (This adds thickness to the bulletin board, so you can push tacks into it.) Lay books on top of the glued coasters to prevent the cork from curling. Allow to dry overnight.
3. Paint the edges of the coasters; let dry.
4. Referring to the photo, adhere the coasters to the wall with removable wall-mounting stickers. Mix and match the color placement in a pleasing pattern.