You will need:
- an envelope
- paper (old magazine pages, calender pages, pages out of photography books, children's books, interesting advertisements, Maps, sheet-music etc, anything interesting you can find that would fit an envelope. )
- glue (Make sure it is paper glue. I have tried different glue sticks and some came undone after drying)
- a straight edge
Get your Envelope and carefully open up the "seams" until you have one flat piece of paper. If you can get your hands on a "good" envelope that's great. They tend to be a bit more stable then the cheapo kind. It's easier to open them up and they stand up to being used as a template a bit better.
You could trace the opened envelope on a bit of light cardboard, so you will have a bit more substantial template. Try different size envelopes and different styles!