Being where I work I do not have to buy the envelopes but I do have to modify the ones . so here is my process. this is the size of envelope I start with as you can see it is 7 1/5 inches wide and about 10 inches in length. I then place the painting on top and make a small mark to know where to make my cuts. I also rip off the fold over flap for a template. After marking out my fold over flap as shown here . I cut along the line and then cut away the second layer on one side .