I put a thickness of tape at the width of the binding that I need to sew, so that it is straight.
On those corners, fold it like the diagrams say. Then check is the fold at the top right on the edge of the quilt, not over and not under. Where the binding is overlapped are those edges right on top of each other. Is the binding that is going down the new side of the quilt straight with that edge.
Do you have a friend that does good corners that can make you fold yours in front of her to see how you are doing it?
Then there is the subject of who in the world cares or even notices if those corners are square. If it is being hung in a quilt shop or a quilt show, possibly, but that is the least noticed part of a quilt
Done is best