Step 1. Measure the width and length (from tip of flap to the bottom) of envelope.
Step 2. Cut pattern paper slightly smaller in width ONLY and place inside envelope.
Step 3. Flip envelope over and lightly trace the edge of the flap onto your paper. I usually skip this part and just start cutting but for this tutorial I marked it in red and removed the paper from the envie so that you can see where I cut.
Step 4. Remove pattern paper from envelope - cut where you marked the flaps and trim 1/2 inch off the bottom.
Step 5. Slide your pattern paper back into your envelope. Fold flap over and adhere paper to the inside of the flap ONLY.
And voila! You should have a perfect fit. By the way, this is a great way to use up some of that old outdated paper that you have buried in your stash!
If you found this tutorial to be helpful, please let me know by leaving a comment.