You can buy a plastic template or take apart an existing envelope and trace it, but really I actually prefer to make them without the template because it is easier. To do this, you:
1. Fold the edges on the longest sides in about an inch
2. Fold the top part of the page into a triangle, by folding in the sides. This is your closing flap.
3. From the bottom of the page, fold up a third of the way to make the envelope part, and fold the triangle flap down on top. Easy Peasy!
(Here is the front of the envelope.)