To create an envelope out of 8.5 x 11 paper, start by folding the paper in half, touching one 8.5-inch side of the paper to the other. Crease, then unfold. Fold the bottom two corners upward until they touch the line left by the first fold; crease, and keep folded. The paper will have a rectangle on the top of the original crease and be triangular on the bottom.
To complete the origami-styled envelope, fold the left side of the paper inward until it touches the center crease. Repeat with the right side. Now, the bottom should look like a diamond.
Fold the diamond in half, bringing the bottom up to the top. Crease, then unfold. Make two more creases along the top right and left sides of the diamond shape.
Slip your fingers in the left and right sides of the slit down the middle of the diamond, and pull outward. The diamond will unfold and create the rectangular base of the envelope.
Fold the top two corners toward the center to create the flap. To use the envelope for mailing a letter, use tape to seal the flap and to keep the envelope from unfolding during its transit.