Place them right side together and sew all four sides, leaving a few inches open for turning right side out.
I didn't add a button, instead I cut a tiny strip of sticky Velcro and placed it on the underside of the envelope flap and the bottom of the envelope pocket.
There you have it, a cute fabric Gift Card Envelope. A cute way to dress up an otherwise ugly gift to give. :)
Here's a shot of it closed and ready to go. All I need to do is add a little tag.