When I posted my mail art envelopes a while back (see it here), a reader asked if I had a tutorial for it. Well, no, but I came up with this step-by-step to my process (which isn't necessarily the correct method; if there is one). It's very easy, and the sort of project where there is no right or wrong (unless you're the USPS-- more on that later).
First-- gather supplies. I used acrylic paint because that's what I have. I would think craft paint would work okay too but haven't tried it myself. Paint brushes. A palette (i.e. paper plate), water can (washed out juice can works fine), paper towel to dry off your brushes, and magazine pages. The higher end type magazines with nicer paper is what I used-- not sure cheap stuff would turn out, might ripple.
This is how I started. No reason. No formula. No goal. Just paint like a little kid. Personally, I like how the magazine image and/or text shows through. Keep in mind you can't work too wet or the paper won't hold it, and you'll have a mess.
This is what I ended up with for no reason whatsoever. Don't get too hung up on a design since some of it will be cut away and covered in the end, and folding it up totally changes the way it looks too. Allow it to dry on a flat surface.
And if your palette doesn't look something like this when you're done, you're way too neat when it comes to painting.
Use a "real" envelope as a template. Just carefully unfold it. Trace around it on the back (unpainted) side of your magazine page and cut it out.
Now, simply fold in all four flaps.
Glue the bottom flap to the side flaps (in back). A glue stick worked well for me. Check your original (template) envelope to make sure you don't glue areas that should be loose-- you do want to be able to get something in there to send!
From here on out, it's totally up to you. Leave it, embellish it a little or a lot. I used scrapbook paper for address and return address areas. Just glued the edges with craft glue and used rubber cement in the middles. And I didn't measure these or any mundane step like that.
Here it is all finished. Why windows? I just spied them in a magazine and thought the colors went well with the envelope. The little text banner and the "TO" are scrapbooking stickers.
A couple notes-- I don't put any sort of adhesive on the upper flap. When it comes time to mail, I glue stick it shut. You could also tape or use stickers. If you do intend to mail yours, make sure it's not a weird size the post office will frown upon. Square envelopes take extra postage; there are restrictions on too big and too small so keep those things in mind. Ask your postal clerk or look it up online if you're unsure. I have mailed several so far with no trouble.