Study up on which gases are used, how they are safely stored and handled, how they are regulated and controlled for gas welding, safety equipment required, gas welding materials needed, which metals can and cannot be gas welded, and any other pertinent topics; to ensure a thorough understanding of the principles behind gas welding.
Attend vocational or trade school programs that teach how to gas weld metals. Programs vary by school, but most offer gas welding theory along with the hands-on experience. Learn how to handle acetylene and oxygen cylinders; how they need to be secured and protected; how to attach a gas welding torch, hoses, and regulators; how to set gas pressures for clean, safe gas welding.
Practice gas welding hands-on as much as possible. Be sure to follow all safety and health guidelines and instructions, and practice joining various metal scraps, to help develop a technique that will work best for you. Forming and flowing a gas weld "puddle" smoothly and prettily is no easy task; it must be mastered through a lot of practice and even mistakes.
Fabricate something you want or need, using materials that can be gas welded together, as a project to help you learn more about gas welding effects on assemblies and structures. Welding metals together always introduces metallurgical changes, such as expansion, shrinkage, twisting, hardening, annealing, and more. Creating a test project will offer knowledge that books and test samples cannot.