Here's a video I found that was helpful to me for Gas welding... It's an Oldie... but it takes you from NOVICE to welding a cluster of tubes...
Also great read: Aircraft Welding and Steel Tube Fabrication Part 1-9
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-1
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-2
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-3
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-4
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-5
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-6
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-7
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-8
https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/tubing/aircraft-welding-and-steel-tube-fabrication-part-9