Fit-up is 80% of the weld: the clamps, magnets and tricks that make joints square

Buy or make: two 12" speed squares, four welding magnets (the cheap arrow ones are fine), a pair of 90° corner clamps, and a flat surface you trust. Tack in a sequence that pins the geometry (opposite corners), re-measure diagonals, then weld. Expect joints to pull toward the weld side about 1 to 2 degrees; pre-set the angle slightly open or weld opposite sides alternately.

On thin tubing, a 1/16" gap vs no gap is the difference between a weld and a blow-through. Shim with a hacksaw blade for a consistent gap.