I'm refinishing one too. This is my second.
First, you should sand down to bare metal. Prime, and then paint. I'm taking some liberties here and assuming you already know how to paint metal.
You've got several choices when it comes to paint. I think wrinkle paint looks nice, but unless you do it right, it can be a big pain.
Rust-O-Leum matte black looks nice too, but not as authentic.
The door I'm working on right now I've decided to have it sandblasted, and then powder coated (which is what the original finish is) There's several types of powder coating, I recommend 'texture finish.'
That's pretty much it, in a nutshell.