What is the best way to square up fabric? Trying to cut some yardage into fat quarters and some of it is way off

Actually, the best way to really stay on grain is to (gently) tear it. But, that said, sometimes it stretches when torn. However, it squares up nicely when washed.

I cut a half yard then cut into half for a FQ. Lay the fabric out, a bit more than a yard (or half yard, whichever) then fold so the selvedges are together. Hold it up and adjust the selvedge edges until it hangs straight a the fold. There may be as much as a couple inches off on the raw edges. Then lay it down, bring the folded edge up close to the selvedges, take the ruler and align the folds (both of them) so they are straight across on the top and the bottom. Cut a straight piece off the edge.

If you Google "how to cut fabric straight" you will get lots of videos and tutorials.

By the way, be sure to cut off the selvedges before cutting your pieces, send the selvedges to me - I make stuff with them. (that's a joke!).