There is a knack to welding light gauge tubing so it doesn't twist but it takes years to master, I'm 45 and been faffing about welding stuff together since I left school.

Clamping doesn't work and cutting to dead on sizes cause problems as does cutting angles spot on....just because you cut two pieces of tube at 45 degrees doesn't mean you will end up with spotty dog 90 degree once welded.