Make sure your selvages are both on the same straight on a line on the mat before any cutting. The raw edge will likely not be straigth ...make a trim cut , cutting on the vertical ( 90 degrees from the salvege). To check to see if you are ready to cut project strips , both the selvage and the raw edge should line up perfectly on lines on the cutting mat , or a perfect 90 degrees( selvage to raw edge) .
This site has two great videos, the second one is for cutting.
http://www.quiltfabric.com/