What is the best way to cut plaid fabric for borders so the lines are straight? I want to do ~ 6 inch borders and I want the lines to be perfectly straight. I am using expensive fabric and don't want to mess it up. I"ve never used plaid fabric before.

pre-wash and starch---a light spritz of starch-press, spritz again...a couple light layers of starch are better than one heavy layer...then line it up very carefully on your cutting mat and line your ruler up with the 1/4" line along one of the lines of the plaid and cut...(doing it this way gives you the straight line of the fabric to follow when you sew---helping with that whole...straight thing :)

If the plaid is printed straight, you'll have no problem lining up the plaid with your ruler. Check with your ruler along the edge, sometimes the plaid is printed at an angle and you have to choose between the straight of the grain or the straight of the plaid. Good luck!

I read in Fons & Porter that they cut theirs with a scissors carefully on 1 of the lines of the plaid to keep the pattern straight.

That is the way I have been doing it. Yes it is time consuming, but well worth the effort. I have yet to see a perfectly straight plaid or border fabric.