Step 1: Main body
Cut off several pieces of duck tape and stick them to your cutting board, so that you have a square that is 31cm x 22cm. Then cut back two strips from the top, 3cm x 11cm as shown in the diagram. Once you have done this, carefully peel the whole thing off you cutting board, and set it aside.
Repeat the same procedure, only this time you need a square 37cm x 22cm. At one end, trim the corners 3cm in from the corner point on the top and side, and on the other end, 6cm in from the top and side of the corner, as shown in the second diagram. Again, carefully peel the whole thing off the cutting board.
Now comes the tricky part. You need to align these two pieces, sticky sides facing and stick them together smoothly. The widest parts of both pieces should align; the narrowest end of the first piece should dissect the longer trimmed corner pieces as shown in the third diagram. Be very careful here; once duck tape meets duck tape sticky side, nothing will part it!!! This is the most crucial stage.
With your knife, make a cut along the sticky part of the second fabric, 11cm down from where the corners meet, so that you can fold it over the first piece. Fold over the left and right ends and smooth down. If you make a better job than me, it will look nice and neat!