How do you cut the bottoms off of old bottles?

Help! There are so many projects that I have in my old head (I'm a senior crafter 75 year young) that involves cutting the bottoms off old bottles. It doesn't have to be a smooth cut, because it will be used only for decorations only.
I love it when I can help younger people with things that I myself had a hard time dealing with year ago
I love this site, because everyone seems to be so friendly and helpful, so I give a big Thank You ahead of time
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