Cutting fabric for the first time after washing and ironing can be tricky especially if they were not careful when cutting it at the store you purchased it from. For me, I start with folding the fabric in half lining up the selvedge edges. I basically hold the folded edge in one hand and the selvedges edges in the other hand. I then move
one of the selvedge ends up or down parallel to the other edge until the fabric hangs nicely without any bulges or wrinkles. This is when you can really see how well the fabric was cut or not. Sometimes I've had to trim off 1/8 yrd in order to make my fabric straight. At this point, I carefully lay my fabric on my cutting mat and cut it from fold to selvedge using my rotary cutter and long ruler. Now I know my fabric is straight and I don't have to worry about crooked corners later.
Here's a link to a youtube video which may help:
http://www.youtube.com/watch?v=C_euef3T7b8