Dumb question

I'm trying to make curtains. I was nearly done, until I realized that the two vertical sides did not match up. I tore out the stitches. Not sure if this proper protocol, but I lined up the selvedge edges, then trimming the other edges straight. Or so I thought. I pressed all the hems, only to discover that the vertical sides were again not parallel.

What am I doing wrong? What should I do so all the edges are parallel/perpendicular?

I've totally done the same thing...arghh.  It's not a dumb question either--curtains take a lot of yardage and it's hard to be accurate without some doing.

Invest in a big cardboard sewing grid like this one: http://www.createforless.com/Dritz+Superboard+Cutting+Board+40x+72/pid14297.aspx?SI=e9273511-d33c-42ec-8f8b-50d7e5616b83

I got mine at Joann's with a coupon and it was worth the money.  Just match the selvedge up with one of the straight lines and pin along that line and cut where it's not perpendicular to that line using the aid of the grid.

Thank you for that suggestion! I think it'll do the trick. I was getting so discouraged, because curtains seem to be such a simple thing to sew and yet I'm having so much trouble getting the edges to match up right!