This envelope is made using 8.5 x 11" paper (I used cardstock--it was trickier, but it came out ok).
Step 1: Fold the letter in half, length-wise
Step 2: Open up the letter and fold the outer two edges against the center fold you just made. You will now have four equal-sized length-wise sections, with a center fold and two "outer folds."
Tip: Because I had torn the edges, by the time I got to the end, some of the letter was peeking through the crack on the back of the envelope. For this reason, I now overlapped the outer edges in the center just slightly (you can see this in Step 6 and the final product).
Step 3: Open up the letter again and fold in the bottom two corners to make triangles. The bottom of the corners should be lined up against the outer folds, not the center fold.
Step 4: Keeping the triangle corners folded, close the letter again at the outer folds (Step 2).
Step 5: Fold down the top right and top left corners of the letter against the center line, creating an arrow shape.
Step 6: Fold down the top of the arrow, making an upside-down triangle.
Step 7: Turn the letter over, which will make it look like a rectangle.
Step 8: Fold the bottom of the letter up.
Tip: The original instructions say to fold roughly a third of the your letter, but I couldn't be sure and it might be different with cardstock vs. paper. To help estimate for this step and Step 9, I did not crease that fold yet, but move on to Step 9 while holding the bottom fold in place.
Step 9: Fold the top of your letter down, making the previously upside-down triangle on the other side now right-side up.
Tip: Like in Step 8, I couldn't be sure that the top and bottom parts would line up well enough to not have a gap peeking through, so I didn't crease them yet in the first picture. The Step 8 and Step 9 folds are creased in the photo on the right, showing how far down the top fold should be to cover up the V-shaped gap of the bottom fold that you see in Step 8 (sorry the photos look the same, and I couldn't get the second photo to post with it rotated properly).
Step 10: Pinch the bottom corners of the triangle and slide them into bottom fold of the letter from Step 8. This was why I estimated the placement before creasing the folds--if I had not folded the top and bottom at good distances in relation to each other in Steps 8 & 9, there would be a gap left like you see in the second (right) picture if you don't have enough of the top/triangle fold to push into the bottom fold. I hope that makes sense!
The third picture is of the top triangle pushed completely into the bottom.
Final Product: Back of the envelope. This is where you could see text from the letter peeking through the crack if I had not overlapped the outer edges just a little bit in Step 2.