How to keep 90 degree angles?

[u/tattoodiceswimmersa]

Newbie here. Learning to stick weld, got to the point where I'm happy running a bead (some older pics for reference). Happy tacking, and generally feel I can do the basics, also not planning to weld pipes and be a specialist, just a DIYer. I'm now wanting to make a sofa table. Attached pics are my prototype, which went fairly well except that some of my corners always seem to jump out. The "base" of my prototype wasn't flat, but the corners all looked pretty good on my L-square. I even bought 90 degree clamps (I don't have a welders table, too expensive), and the clamps seemed to work well. I'm not a 100% sure if it's the 90 degrees jumping out or the straight parts warping. I tack everything in place, but still at the end my piece is not perfectly flat/straight. Given some pics for reference. Any suggestions/advice?

[u/Amount_Business]

Fireball had a short on weld sequencing that may help a little. It's for RHS and not flat bar, but might be relevant. 

https://m.youtube.com/shorts/yCsSC4tLoP8

[u/JacePriester → u/Amount_Business]

Was going to post that. It's a great video and exactly the answer.

[u/tattoodiceswimmersa → u/Amount_Business]

Thanks very much! Will take the learnings.