cheap sheet glass first.
you do not need to cut the score line very deep. using too much pressure could prevent the glass from breaking evenly.
be sure to cut from one edge to the other. it is not possible to break the glass evenly if the cut is not from edge to edge.
do not cut over a previously cut score line if you make a mistake. you will need to cut another score line. doing so will ruin your cutter.
position the glass. line up the score line with the edge of the table. the largest piece of glass should rest on the table and the smaller piece should be off the 