You will need:
Cut 2 pieces of poster board to size mine was 10'' x 8" but you could go larger. Remember you will just need more pins. Once you have 2 pieces the same size glue the two together.
Now if you have board already marked with a grid you can use that, I didn't so using a ruler and pencil I marked 1" squares across the board.
Next you take a pin and press it into the first dot, remove it, put a small dot of glue into (on top) of dot and then reinsert the pin. Repeat for every dot.
Finally I taped a duct tape boarder around the board and left the pins to dry. Once it was dry I checked all the pins were secure (I had to re-glue a couple) and then I gave it to D to play with. To be honest I was surprised at how long it kept him busy, he enjoyed stretching the elastic bands as far as he could and in his orderly way made a nice straight line pattern, you can see the concentration on his face.