First, make sure you are in a well-ventilated area. You will be burning through plastic soon and it is not a good idea to breathe in those fumes.



Next, use your lighter or another flame source to heat up a sharp blade. With the cap removed use your hot blade to cut from the top of the bottle to the mark that you made earlier. Cut another parallel line down to this same depth, the distance between these should be slightly bigger than the smallest part of the neck of your safety razor. In my case the width was about one centimeter. You will likely have to re-heat your sharp blade multiple times to make all necessary cuts.



Then cut from the bottom of both of your first two slices so that they meet each other and remove this whole rectangular section from the bottle.