Align your rectangular tubes in the exact position in which you plan to weld them and clamp them in that position.
Create small "tack" welds with your welder, where you plan to weld. Tack welds should be small and evenly spaced. Tack welds hold your tubes in place when you are producing your continuous weld bead, avoiding warping. Allow your tack welds to cool for several minutes.
Produce a continuous weld bead that connects all of your tack welds. Move as quickly as possible when producing your continuous bead. Move your entire body, not just your hand, to keep the angle at which you are welding constant.