Like the concrete slab your house sits on, the sheetmetal is the foundation for your paintjob, and it needs to be as straight as possible. Metalworking takes practice. Metal masters like Ron Covell have studied for decades to perfect their skills, and their craftsmanship shows their tenure. Understanding metal and how it reacts is important, and you can get a good grasp of basic action/reaction techniques and metal forming with some simple tools. You can usually go by any autobody shop in the country and get "take off" panels that are damaged, and then practice hammer-and-dolly techniques until your hands hurt. This will give you a good idea of how sheetmetal reacts, and what's required to repair and reshape it.