I'm still working on this, a project in progress. I know there's a simple and obvious way to make a good fold-and-mail letter, but I haven't quite put it into practice yet.
Observe my crude pattern on the left. Ideally, the bottom half of it should fold up into the top half, and the flaps should not only enclose it securely but the corners should run flush against each other as well.
Can't say that happened exactly, though it was close. I'm making these as part of an interactive project from Facebook--actually, I probably mentioned it back in April. Too lazy to check right now. The gist was this: I posted a meme that solicited five people to sign up (who would also repost the meme). For each of these five people I had to create and send some kind of creative project, and since all I know is stationery, I thought I'd make some postcards and a fold-and-mail letter for them.
It's just that I didn't think this thing through. Each fold-and-mail item is one exterior and two sheets of paper, each sheet being half of one regular piece of paper. I glued in the sheets at the top, just below the large upper flap, so you could fold the exterior and the sheet-halves in their middle, then seal it down with the three flaps.
I think, to really help the folding and the edges meeting up nicely, one might trip about 1/8" off two perpendicular sides of the sheets of paper, but not the exterior. This should give the paper enough room to bulk up with the fold and not create extra space before the flaps can fold in.
But here's my pattern, if anyone else thinks they can use it. This isn't the cardstock envelope I promised back in December. I'm sorry about procrastinating on that thing, but I haven't forgotten about it, and maybe crafting and scanning my design for this project will motivate me to perfect and upload the envelope pattern.
Anyway, the flaps seal with double-sided window insulation tape. I trimmed it in half lengthwise to be narrow enough for the side flaps--otherwise, you can't beat it for adhesion. Leave the backing on for the letter-sender to peel off just prior to mailing. Better solution than sending everyone a glue-stick they'll use once and lose. And the address labels are my own design: I carved out the border in linoleum, stamped it onto parchment, then cut it out closely and glued it to the front of each exterior.