Step one: TURN YOUR PAPER OVER so the gritty side is facing down. Mark any size and shape you need on this smooth paper side.
Step two: Use your scribe along a straight edge (or maybe a template if you want to make curves or complex shapes) and SCORE the paper. Don't try to go through, we don't want to dull the scribe with grit.
ta-dah! You'll see that the paper weakens so it will tear with a very clean edge.