What kind of results are you seeking? If you want to simply fill in the scratches with touch up paint, I can give you a few tips, but if you're looking for a perfect "factory" finish, you may need to work with an experienced auto painter (or body shop) to sand, fill, prime, paint, rub out etc.

If you plan to touch-up, here are a few tips that have helped me. First, get a couple of good quality artist's brushes from an art or craft supply store. You can get brushes that are about as narrow as the scratch itself, which is much better than the brush in the cap of the touch up paint! Get some lacquer thinner to thin the touch-up paints (keeps it from drying too quickly) and to clean your brushes. You may want to try filling the crack with paint provided that it's isn't all the way down to bare metal and has begun to rust. (If it's rusting, you'll have to get rid of the rust, prime and then paint).

Go slow, build up the coats in the crack with several coats, and see if the results satisfy you. If you try and don't like the results, you can always take it to a pro who will sand it all off, anyway.