Roll-Up Board Game / Card Game Surface

Step 10 Fold in and pin the second crease

Now fold along the line of the second crease, tucking it under so that the edge meets the first crease, and pin it in place.  (The picture shows a close-up of this process half-way along one side.  Note that when I took this picture, I'd accidentally failed to put the pin through the bottom layer - the pins should go through all three!)