Larger projects such as cabinets or shelves can often be challenging to "square up". These handy tips will help you square up even the largest projects.
A quick trick to ensuring boxes, shelves, and other projects are perfectly square, is to measure from opposite corners. When the two measurements are the same, the case is perfectly square.
3-4-5 Triangle Method
This trick is used often in the construction industry for setting up foundations and other large projects. To use this method you would measure and mark a point 3' from the corner. Next measure 4' in the other direction. Stretch a tape measure between these two points. When the distance between them is 5' the corner is square.