First, print out the preceding design.
Then cut it out. Color it if you want.
Then fold it together at the head and back. Glue the sides of the head together and put a tape loop inside the neck. Fold the leg flaps toward each other, with the flaps on the left foot bending toward the flaps on the right feet in a open "V" shape. Overlap them and fasten with staples, tape, or glue. Iguanodon is basically done at this point, but more artistic paleontogists can bend creases into the body , outlining the hips and shoulders and showing where the miuscles might be.