I’d strongly recommend you level first, then work on square. Here’s how I get my machine level. This will give you a reference for perpendicularity and you can then work from there (you now have a x/y plane, and can reference the Z axis. Once you have Z, you can do X and Y which as you note are interlinked.)
There are some discussions here about squaring, and a step by step tutorial, somewhere, I can’t seem to find it either. @WillAdams I think had one?