Practical welding can involve welding in all kinds of awkward positions,
like upside down, head first, under a pipe in a ditch and lots of other
strange positions depending on what you are doing. You can learn how to
weld in a class or at the bench. You learn how to be a welder by
applying that knowledge in the field. You learn how to tackle new
situations by asking knowledgeable folks, watching them, or using your
head, trying something and seeing how it works. As far as welding a
fabric covered airframe, you remove nearby flammable materials and/or
cover them with fire resistant material, and then have a firewatch
standing by with an extingiusher. Heck, plumbers sweat pipes in walls
with wood studs all the time. It is pretty much the same concept.