Things You'll Need
Instructions
Print Bruno van de Casteele's D20 pattern (see Resources). Cut it out carefully with scissors or a craft knife.
Make a crease along all of the lines marking the flaps and the divisions between the triangles.
Attach the flap on side 14 to side 20 using glue or double-sided tape. (If you're using glue, wait for the glue to dry between each side before moving on to the next.)
Attach the flap on side 15 to side 5. Your die should be in the proper shape, but with a large, jagged hole in it. One side of the hole should have four pieces without flaps (17, 13, 9 and 2), while the other side should have three pieces with two flaps each (3, 19 and 12) and two pieces with one free flap each (7 and 15).
Attach each piece without flaps to the two flaps surrounding it. This will get progressively more difficult, because when you reach the last piece, you won't be able to press the flap and piece together from the inside of the die, and when you reach the second-to-last piece you'll have a difficult time maneuvering.