If cutting the edge vertical, hold the glass so that the top of the lead is aligned with your mark and using a sharp blade, slice into the lead. Lightly tap the blade on either side of the cut to release any excess lead. If cutting the edge horizontal, place the glass so that the top of the lead is aligned with your mark and make sure that it is stable. Using a curved knife or razor, slice lightly into the lead creating a V-shape. Tap lightly on either side of each cut to release any excess lead.