You can use a file or sandpaper to do this but if you want a uniform finish that is quick to achieve a cup bur is a great tool. You can buy them attached to wooden dowels or you can just buy the burs and put them in a pin vise or rotary tool. You can even just turn the bur in your fingers. Burs are designed to cut in 1 direction. If you are looking into the cup of the bur the cutting direction is counterclockwise. If you are looking at the back of the bur the cutting direction is clockwise.