Step 1: Materials Collection
Here you see an excellent source for gathering raw materials. For envelopes, you'll want paper at least 8-1/2 x 11", and bigger is better. You will also need scissors and scotch tape. In the final step, you will want a Sharpie pen, or white paper to tape onto your envelope, for addressing purposes.
Step 2: Image Selection & Initial Folds
Begin by folding your chosen image on the diagonal. The first fold will create the bottom of your envelope's front. You'll want to fold over more than you might think, in order to make the envelope wide enough to hold most paper or cards.
Step 3: Fold in the Sides
Details: The edges should overlap. Make sure you have enough paper on each side to cover up the envelope's future contents. A common mistake is to make one side too short, as the side bits are uneven at this point.
Then, as in origami, unfold it and reverse-fold each of the bottom corners. This makes it look more like an envelope, with the middle fold now on top.
Step 4: Complete the Shape and Tape It Down
Tape the sides down. Also tape over the edges. You want it to be as strong as possible because the post office workers will be so fascinated with your envelope that they'll handle it a lot.
Step 5: Final Steps & Voila!
To address it, you can do one of the following:
1) use a Sharpie pen, glitter pen, or the like on a blank portion of your image. (Just be sure to pick something water resistant and extremely visible)
2) glue or tape on a piece of white paper that's been cut into an interesting shape to serve as an address label
3) Use one of those sticky tags for same purpose
Add your return address and a stamp. Tape shut and send. And await the oohs and aahs from your correspondents who've gotten so used to e-mail only that they'll be wowed by receiving this work of art, uniquely made just for them, in the post.