If you don’t have yarn available, you can use any thick cotton string. At the location of where you would like the bottle to break, wrap a piece of yarn around the bottle 3-5 times. Tie the ends together and cut off any excess string.
 Slide the yarn off the end of the bottle, and place it in a small dish or lid. Pour a bit of nail polish remover or straight-up acetone over the yarn until it is completely soaked. You can pour the excess acetone back into the bottle afterwards.
 Take the yarn and replace it back on the bottle in the exact location you want it to break. Try to make sure that the loops of yarn are placed close together and are tight so that you get a level, clean break.
 Use a match or a lighter to catch the yarn (on the bottle) on fire. Rotate the bottle slowly so that the yarn burns at a consistent speed around the entire bottle.
 Have your sink or a pot filled with cold water - you can add ice as well if you would like. Wait until the fire on the yarn has burnt out, and then stick the end of the bottle with the yarn directly into the water. The bottle should break cleanly off where the yarn was wrapped around it.
 Use a rough sandpaper to smooth down the jagged edges on the broken section of the bottle. When you have removed any sharp corners, switch to a fine grit sandpaper to give the bottle a smooth, soft finish. You’re done!
Wrap your yarn.
Soak the yarn in acetone.
Wrap the bottle again.
Light the yarn on fire.
Dunk the bottle in cold water.
Sand the edges.
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