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.