Wear safety glasses, gloves, and long sleeves
Clean and dry the bottle thoroughly
Mark the cut line with a glass marker or tape
Score the bottle once with a glass cutter or bottle cutter
Heat the scored line evenly with boiling water, a candle, or a torch
Cool the scored line quickly with ice water
Repeat heating and cooling until the bottle separates
Sand the cut edge with wet/dry sandpaper or a diamond file
Rinse and inspect the edge for sharp spots
Dispose of broken glass safely