Underneath a bottle cutter, you can cradle the bottle on a couple of rubber rollers, which are adjustable. The cutter is also movable, so you can decide where you want to cut the bottle in half. Once you have the cutting head adjusted, simply apply pressure to the bottle while rotating it until you have completed 360 degrees around. That’s all there is to the cutting part.
Keep in mind, you cannot force the bottle parts apart as you can with flat glass. The easiest way to do it is to have two different water baths close by. I use small plastic trash cans: one filled with hot water (boil it and fill) and the other with ice water.
First, submerge the bottle in the hot water for a few minutes until the whole bottle is heated up. Then pull it out and submerge it in the ice water. This shocks the glass in such a way that it breaks apart along the cut line. It takes a little experimenting with the amount of time since you never know how thin or thick a particular bottle is. Some of the thin ones break apart quickly, while thick bottles can take a little bit to heat up enough before they break in the ice water.
-Ruth Dobbins, Professional Glass Consultants