After destroying several bottles i found the right way to use this homemade tool. First we cut a line around the bottle by turning it two rounds.Then we put along the line hot water and then icy water.We repeat 2 or three times and the item is
ready. You can see here how this tool works. When you are surfing the internet you can find a lot of constructions witch are made from professionals.I made this tool in two hours but I spend alot of time to find the right way to cut the bottle.The last two photos are works that are made from proffesionals (not mine) with similar tool.
WATCH THE TOOL IN USE HERE.