You can download a form from the wildlifedepartment.com web site, or in my case, I had the Dr. hand write that I could not pull a standard bow any longer due to a ruptured bicep tendon and would like to see me use a crossbow. He signed it, and I've shown it to every ranger in the counties that I hunt in, and they all said it was good enough for them. We took it to a print shop where they shrunk it down to credit card size, and laminated it. I keep the original in a file, and the other in my wallet.
It must be a permanent injury. They don't issue temporary permits.