How do you make a foam pyramid?

Start with a large foam ball. Calculate what parts of the ball you need to remove to make the pyramid, using your geometrical formula. Plot the points on the ball that represent the pyramid and mark them. From the points on the base, make a line connecting them around; this will be come your base. Now cut the ball with a fine wire or jig saw. Repeat the process for each face of the pryramid.