Gunsmiths must understand not only how to properly handle and use many different types of guns, but they must also have knowledge of machine tool processes, ballistics, metallurgy and woodworking. The knowledge and skills needed to be a quality gunsmith are learned over time.
Draw from skills you may already have. Ideally, you should already be experienced with metal working and have a deep affection for weaponry. Being able to understand the "human aspects" of gunsmithing, such as balance, sights and grip is also very important.
Educate yourself. A formal education through an apprenticeship or attending an accredited gunsmith school is critical and combining the two is optimal. By combining the two, you will learn about gunsmithing and the behind the scenes business aspects of the job. Consider spending at least a few years working for a professional gunsmith before trying to hang your own shingle.
Get work experience. Supplement your education with real world experience. Although you will learn the technical ins and outs of gunsmithing through an educational program, you might not get the one-on-one attention and words of wisdom you will gain from actually working in the field.
Keep your day job. Gunsmithing is a highly specialized career that does not always start out as a lucrative one. It is very possible that you will need to hold a full-time job and gunsmith on the side. Sporting good stores often have opportunities for professional gunsmiths (though, depending on where you live, these jobs might be seasonal). These jobs are a great way to learn and build a contact list for when you do go out on your own.