Steps

1. Write the word BACK on one side of the paper in large letters.
2. Make a crease through the center of paper, length-wise, so that the "back side" is inwards. Flatten back out.
3. Make a crease through the center of paper, width-wise, on the same side. Flatten back out.
4. Position the paper in front of you in a landscape orientation and so that the word BACK is showing. The following directions will refer to the paper in terms of this current orientation. (e.g. the short side now is the vertical)
5. Fold up away from you approximately 1/6 of the way up the vertical from the bottom.
6. Make a crease on the lower left corner as if making a right triangle that's about 1/2 inch in width and height. Do the same for the lower right corner. These creases will serve as guides in the next step.
7. Using the crease from step 6 as a guide, take the left side and make a vertical fold inwards so that the fold is parallel to the side and the bottom of the fold line meets the bottom end of the diagonal of the triangle from step 6. Do the same for the right side.
8. This step is the same as step 6, but this time you will be making actual folds (not just creases) on the lower corners to make right triangles whose leg meets the vertical fold made in step 7.
9. Similar to step 7, use the bottom end of the diagonals as guides to make vertical folds inwards parallel to the sides.
10. This step is similar to step 8. Make folds on the top corners that form right triangles whose width and height are about an inch each.
11. Make a fold downwards toward you about 2/5 of the way down the vertical and insert the "tip" of the envelope cover into the bottom inlet.
12. Flip the paper over and draw a picture of where you'd like the envelope/letter to be sent.