Step 1: You will need to cut out a rectangle 27 x 16cm.

Step 2: Cut the corner off the rectangle 10.5cm from the top on the height to 18cm on the width __________

Step 3: Create score lines in the places indicated - - - - -

Step 4:  Now fold the scored pieces as indicated

Mountain Fold  (Shorter score line) the paper is folded behind itself with the crease at the top. 

Valley Fold (Longer score line) the crease is at the bottom and the paper is folded into itself. The paper should form a V. 

Step 5: Decorate as required.