Use a small amount of corn flour underneath the paste (so that it doesn't cling to the board). Cut out squares with a square cutter as shown in the photo (I used a 10 mm cutter). Make sure you press the cutter firmly all the way to the board. Move it around the board a bit as if you were trying to rub the edge of the cutter against the board - that way you will get a clan cut. You can also clean the edges with a smaller square cutter if there's need (press it to the corners).