How do I make a script that makes a person fly when they double jump?

I want to try to make a script that flies when a person double jumps and if they double jump again(while flying) they stop flying. I have no idea how to do this at all. I found a free script online and tried to make it off that but when I asked for help it got taken down. Here is the thread if you want to see the script I used:

https://scriptinghelpers.org/questions/69967/why-wont-this-fly-script-i-found-not-work-the-way-i-want-it-to

Please help me out! Anything helps.