Making Paper Boxes tutorial--in fact, I strongly suggest you do that one first if you haven't already, since many of the instructions are the same and it might be a bit more detailed. You may also want to check out my tutorials for Pentagon Boxes and Heartboxes/Favors. I also have added a new size of envelope, 3 x 1/2, on another post!
I LOVE making envelopes! I have created my OWN PATTERN FROM SCRATCH! Though, I will admit--I did NOT invent the actual envelope. So maybe I'm infringing on copyright there...
I have two free patterns for you that you can download from 4shared. They are just two different sizes of envelopes, in a Photoshop document so that the layers are retained. There is one layer, which is the lines of the box, and a second layer which is the background or "shape" of the box. It looks like this:
RealScrappy Envelope Patterns