My next quick tip for you is actually a really important tip for woodworking and cabinetmaking. If you want to build square you need to have a “Square Standard” to go off of when you are marking and cutting parts and pieces to build with. If you are using a square that is not “true” to mark and cut, every step will compound the “out of square” variance and you can end up fighting a losing battle when you get to the end of the build. You can’t square up an box when the sides and top and bottom pairs are all out of whack .
If you’re trying to build a trapezoid, that’s another story.