My wife bought a plastic (crappy over-priced) glass bottle cutter from Amazon to create glasses and bowls from cut glass wine bottles. After trying the gizmo she ordered and trying a variety of techniques on the internet I cam up with this.
The glass cutter fits in a slot cut to hold it tightly in the front piece which slides back and forth to change the placement of the cut.
Place the bottle in the jig and firmly hold it against the backstop and turn it completely around to score the bottle.
When the bottle is scored roll the score over a lit candle to heat it up and place the bottle in a bucket of cold water. The two pieces should snap apart.
Another method of separation is to place the bottle in the sink and pour boiling water over the score line.
Sand the rough edge until smooth.
Wash the pieces thoroughly.