This is what the finished product looks like.
Cut a square for the envelope. Make sure that its big enough to fit the card inside.
Fold the right corner up to the edge of the card. You can leave a little space web you fold it if you want the card to fit loosely.
Do the same to the left corner.
Fold the bottom corner up.
And finally, the top.
Use glue to secure the envelope flaps.
Glued bottom flap.
Add a little bit to the top corner to seal it.