How to make a pin-board- Step 3.

Cut the over hang from the corners.
Fold the corners in. Fold both corners on one side at a time.
Fold the flap up and over to the back, keeping your corners neat. Secure with staples. Proceed to opposite (remaining) side and fold the corners in then fold the flap up and over, pulling tight to prevent wrinkles on the front.
At this stage you should have all four sides secured, with nest edges and corners.
The front should be without wrinkles and be taught across the tile.