Open the flap of a small envelope.  Using a pencil and straightedge, draw the diagonals from each bottom corner to the opposite top corner.  Draw a vertical line connecting the intersection of  the two diagonals and the peak of the envelope flap.  Fold along each of the lines.

Cut along the top of the two diagonals until you reach the intersection of the diagonals.  Discard the top/flap of the envelope.

Can’t draw this one—Use your imagination.

Open up the envelope, flattening the bottom a bit.  Tuck one side of the envelope into the other side of the envelope.

.

You should now have a tetrahedron.  The sides can be decorated by untucking the tetrahedron and drawing on or coloring the exterior sides.