Once long ago I needed a lot of glass tanks for an experiment, and I made them by cutting the tops off of gallon glass jugs (drank a lot of cider that summer). I used a regular wheel glass cutter, and made a jig out of a pair of 2x4 scraps at right angles, such that the base of the bottle rested on one, and the glass cutter was clamped against the other. Rotate the bottle against the cutter to get a good scribe line, then tap the scribe line FROM THE INSIDE OF THE GLASS. That turned out to be the secret. Broke a lot of bottles before I figured that part out. I bent a piece of metal rod so that I could put it through the neck of the bottle and tap against the glass from the inside. Just tap gently and rapidly against the scribe line, working your way around, and after a few taps, a crack will form and start following the tapping around. Don't try to pull at the glass or force the crack, or it will wander instead of following the scribe.
Hope that helps a bit.
A.