Welding Tips and Tricks

Hello
I am new to MIG welding and have been making my own gym equipment using mild steel box section tubing.
Most of the connections for this equipment comprise of 10mm plate welded to 3mm thick box section ends to allow for bolt/nut securement.
After welding up I find that i can not keep the plate from curving inward and therefore the plate no longer sits flat against the face of the connecting piece of box section tubing. I find i have to cut and weld triangle shaped metal pieces to fit into the right angle between plate and box and this works great but there are times when i just want plate welded to box ends for simplicity and aesthetics.
I thought about tacking up then bending the plate in the opposite direction slightly prior to welding but will this method just snap the tacks off?

pics help a lot in cases like this.
there is tricks to help with distortion but your always going to get some, so its a question of how bad and can you compensate for it.

Clamp or tack weld 2 pieces of the 10mm back-to-back. Then weld the flat stock to the tubing. After, cut the tacks off. This helps immensely in preventing the plate from curling.

Pre bending a piece is a method of minimizing and possibly eliminating distortion from welding. Your thinking is correct.
However, this is something that takes some trail and error testing to get it right. How much pre bending that needs to be done depends on several factors. Material thickness, type of material, welding process used, electrode size, arc voltage/amperage, travel speed etc.
You will have to set up some testing and mock situations to get it right.

Other methods for distortion control are: modifying the welding procedure, choosing the appropriate welding sequence, restraining and heat shaping.
Presetting, preheat, backing bars, strong backs, intermittent welding, balanced welding. All details of things listed above that can be used to help weld distortion.

If you can, cut a square hole in the plate. Pass the tube through, weld the other side.