The first three things you need for scratchbuilding are, research, research and more research. Next, you need to be able to scale up or down from line drawings or photographs. If you're truly scratchbuilding a tank rather than kit bashing, you will want to cast repetitive parts, such as track links and road wheels. You'll need the skill to create compound curves by carving a master and press forming styrene over it.
And you'll need a sense of irony and humor. One of the Peter Prionciples of scale modeling is, "As soon as you complete a major scratchbuilding project, someone will release a kit of your project."