Fold the envelope in half so the shorter sides meet, creating a crease in the middle of the envelope. Flatten the envelope open again. Use the ruler to help you draw a diagonal line from one corner to the other. Score the line with scissors and then fold along the line. Repeat the same process with the other corners.
Cut along the lines from the upper corners to the center of the envelope. Discard the top piece that you have removed from the envelope.
Open the envelope and start to fold it along the first fold you made in the middle.
Take the upper left corner of the envelope and tuck it into the inside lower right corner. Place the three-dimensional shape on a flat surface to exhibit the tetrahedron.