Here are some tips.
- Weld the front to rear cross brace on the inside before you add any other tubes. It will be very hard to weld if you don't. You can grind it down a bit before you add the other tubes.
- Add the diagonal cross brace before you add the outer bushing tube.
- Use tubemiter (http://www.ozhpv.org.au/shed/tubemiter.htm), print, mark tubes, then grind.