Accounting for warp while welding precise corner joints
Most work I've seen like this where it needs to be precise employs 2 techniques.

The first of which is a welding Jig, complete with braces, clamps and the like to forcefully hold everything in position. Most of the time this uses a large, heavy welding tables with holes and slots to bolt the braces and jig too. C-Clamps and whatnot are helpful as well.

Additionally, it's not uncommon even after a well produced weld, to check for true and use a torch and tools to relieve the stress, while tweaking the alignment.

TL:DR - Keep it from warping in the first place, but if/when it warps, heat to adjust and de-stress as needed.

Also, it does look like you're welding a bit hot in places. Practice, practice, practice. There's nothing wrong with grinding out your weld, and starting over, or using a REALLY big wrench/hammer/pliers to straighten it out.