Everyone has their own method.
The fit must be good to prevent pulling while welding
Material must be clean inside and out, some builders go as far as to remove the mill finish on the outside of the tube prior to welding, but not a must.
Back gassing, pre heat, chamber welding is left to areospace and nucular reactors. However, one should never weld on a frame that is cold, warm is better, and for that if you can run a torch over the tube and see a shadow move,then the tube is too cold and that shadow is moisture. I have a roofing torch and sweep the tubing on cold days that way the weld does not cool down too quick. Slow welding is the best as the weld area heat goes out further and slowly draws back to the weld. It takes a very very long time to properly weld a chassis together. I've done it often, and still takes me over a week and a bottle of argon, 5 stainless brushes, and a gallon of acetone. Hot days are the worst because you can have any fans running and keep the doors closed and foot traffic to a minimum