Read the rule book. Look at lots of other cars. Use the right material. Make sure the joints fit very tight. Add extra bars , gussets. Tie the cage to the body at the A pillars, B pillars, cowl, any where you can. Tack everything before any final welding. If you have any question on welding, have someone that you have seen their work do the final welding. Designing to drop tha tacked cage thru the floor to gain access to all upper welds is a good idea. Take your time, no hurry. I built and designed my cage myself. It got good comments from the Division 3 tech director who certed it and a few well known chassis guys to boot. I am not a tig welder. I bent most of the tubes, fitted them all and tacked it. Had a professional weld it all up.
Doug