To tear a fabric , make a small snip in the salvage edge enough to cut past the salvage , take a side of the snipped fabric in each hand and tear completley to the other salvage. It will tear along the grain of the fabric. It can be quite surprising how off grain many cuts can be!
I always tear a backing fabric that way it will lay straight. It also helps reduce puckering. If you have a backing off grain , you will be sewing with a slight bias. It also squares off the edge so you have two good corners to start when you do the layering.