If you can weld then easy.

If you can't weld then harder.

Seriously all metal pulls to the weld as you weld, you can allow for this and instead of clamping at say 90 degrees then lean the metal a degree or two and let it pull it into shape.

For frame work it pays to weld up some angle iron triangles that are perfectly square [ that gives you some practice ] and use these to clamp to as you weld. With a lot of clamping and puling and using well designed gussets at corners it's not to bad to correct any mistakes.

Main thing is to check and correct as you go along and not expect the whole job to compensate.