How can I make my woodworking project square?
Having screwed it together, and as importantly made the holes for the screws, you may need to remake one or more pieces of this frame, or choose new locations for the screws.

In some cases, one can put a clap across a diagonal of a frame to draw it into square if it is not racked too badly. Then new parts of the frame can help to keep it all in the desired shape.

Clamp it up carefully BEFORE you put in those screws. Make sure that everything is in its proper place and stays that way. A large part of doing fine woodworking well is in how much care you take in the small stuff.