Here’s what you’ll need:
Start by cutting a rectangle of felt that is as wide as you want the finished envelope to be, and about twice as long. Fold the bottom two-thirds of the rectangle up, crease it, and unfold. Run a line of glue from the crease down each side of the envelope as shown.
Fold the bottom of the envelope up, and press the edges to seal the glue. You can leave the top flap as is, or cut it into a triangle or a fancy design like I did here. Next, cut a heart out of your other color of felt, and glue it onto the back of the top flap as shown.
Cut two pieces of velcro to make the closure for the envelope. Glue one piece to the underside of the top flap, and the other piece onto the envelope. Make sure they meet up when the flap is closed!