Score and use a glass cutter tool.
a string from a cotton mop wraped around a couple of times tie in a knot snip off. slide string off ,Saturate with nail polish remover, and slide back on . Light the string and keep turning so it burns even. When flame begins to die down plunge into a bucket of ice cold water ,glass will crack and pop and make noise and separate lightly sand edges to get sharp edges. You can find this on Pinterest
Charlotte, you need a board for stabilization, a clamp to hold the bottle, a glass cutter that is as thick (wide) as the glass of the bottle, and a towel to hold the bottle and protect you from getting hurt if a bottle breaks. You can use a countertop as the board. If you have a way to spin the bottle in place, great, but you can do without it. The towel is very important, as you will have glass pieces as you cut the bottle. Mark the bottle for cutting with a washable marker or etch it with the cutter. Clamp the bottle or be prepared to hold the bottle down firmly during cutting. When one side is cut, rotate the bottle and cut the other side. Some people prefer to rotate all through the cutting, creating a groove that eventually brings the bottle to breaking. If you have a rotator, this requires less holding down and usually creates a smoother cut. Use sandpaper to smooth the cut edge and the bottle is ready for your use. If you can have someone take photos as you work, other Hometalkers can learn through your experience. Best wishes, 😊
Score the bottle where you want the break to be. Then boil water and pour it slowly along the scored line. It will break cleanly.
So pleased you asked this Charlotte as I want to do it too. Has anyone tried the above methods on a bottle that is not circular as I have a hexagonal one?