My most recent custom order requested some mini thank-you cards with a baby theme. I don't usually make envelopes for my mini cards because they are smaller than the minimum mailable dimensions. Of course, custom orders always push me to do things I normally don't!
Since my mini cards are 3.5 by 2.5 inches, they are the same size as an ACEO (artist's card, editions and originals) or ATC (artist's trading card). So, these envelopes will fit those cards.
Start with a piece of paper that is 4.75 by 6.75 inches. You can cut two of these from an 8.5 x 11 inch paper (North American letter-sized paper). I use paper that is thicker than copy paper but thinner than cardstock. I have no idea what the actual weight is but it's very nice for envelopes. It has weight, but folds easy and is completely opaque so you can't see hints of the contents through the envelope.
Score at 0.5 inches from each side of the 4.75 inch width. Score at 2 inches from each end of the 6.75 inch height. Cut away the top and bottom skinny half-inch flaps, leaving the part in the centre. Angle cut the remaining flap pieces to make them tab shaped. (Click photo for larger view.)
I like to corner round my top and bottom flaps. I think it helps it to look more finished. I used my 2-in-1 corner chomper to do a half inch round on the flap and a quarter inch round on the closed bottom. To seal the envelope, I use tape where the backing is half an inch wide, but the tape is a quarter inch wide. That makes it easier to peel off the backing when it's time to seal the envelope. (That tape is a little pricey though, I have to say!)
Hope you find this tutorial helpful.