Bleeding obvious to many, but I still keep coming across people to whom this is new:

1. Find a short piece of copper pipe. 22mm is good, but 15mm is OK too. Make sure there is no solder on it; paint is OK, but it will stink to high heaven shortly.
2. Cut a slit down the length of the piece. A saw gives me the cleanest edges, while shears tend to leave a slightly more jagged edge. If there is any internal ridge to the pipe, try to cut along there.
3. Anneal the chunk of pipe. Get it up to a nice dull red, then quench in water or air cool as the mood takes you. While this isn't strictly a pipe any more, don't point one end towards you if you do quench it (quenching pipes like this can blast a jet of scalding water and steam).
4. Pickle it.
5. Using fingers (for 22mm pipe) or ring pliers, splay the pipe open. Then flatten it on a bench block, anvil or other flat surface.

You now have a sheet of fairly pure (.999) copper about 0.8 to 0.9mm thick. A 22mm pipe unrolls to about 2 1/2" wide.

Trivially simple, isn't it?