If you want to get exact details, call your airline. But basically, lock the gun in a hard case, have it unloaded (or better yet disassembled so there's no question) and make sure it can be locked and that you have a key or combo to it.
Place it in your luggage (that is again lockable) and go to the counter where you check in the bags. Inform the person at the counter that you wish to declare a firearm in a checked bagged. They'll take it from there (verify that is in indeed unloaded and in a hard locked case, they'll relock it and put a red tag on it, and inform TSA about it).
The one time I did it with Delta it went off without a hitch (FL to AZ and back) but your milage may vary.