Unfold the envelope to create a template; take care when you pull apart the glued seams.
Place template over the back side of the page you plan to use and cut around envelope
Flip the newly cut image right side up and add a piece of masking/washi tape creating area to write address
Cut a piece of paper or use card that coincides with size of template envelope for your insert. Here, I used some scrapbook paper that coordinates with the image.
After you write your note, add it to the new envelope...
...then tape it shut.
As you can see, a variety of looks can be acheived based on the images and note paper chosen, and the recipient will be very pleased to receive the beautiful, unique correspondence!