This section is from the book "Experimental Glass Blowing For Boys", by Carleton J. Lynde. Also available from Amazon: Experimental Glass Blowing for Boys.
The common glass cutter is a small very hard steel wheel mounted on a handle (Fig. 85). Practice with one on a pane of glass: place a ruler on the glass, draw the wheel along the ruler (Fig. 86) with sufficient pressure to scratch the glass, place the under side of the scratch exactly over the edge of the table, and press down on both sides.
Fig. 85. A Glass Cutter