Does this help: https://www.hometalk.com/4307763/how-to-cutting-bottles
Here's another: https://www.hometalk.com/1426408/how-to-cut-a-glass-bottle-in-half-with-yarn-and-fire
try this: https://www.craftylittlegnome.com/cut-a-glass-bottle/
As artist who has done lots of glass fusing with bottle glass I will tell you to ignore the burning string tricks you see on YouTube. I never got a smooth cut with that hack.
Get a good glass cutter which really just scores the glass. You need steady even pressure to scratch in the scoring smoothly. Don't try to deepen the score by retracing because it will be ragged. Then after a single scoring the thermal shock of alternating hot and cold will evenly snap the glass.
If you have a wet tile saw you can try it too.
Bottle cutting kits are inexpensive and readily available. I've seen them for about $10