How do you take your idea and write it into a joke?
First off you have to have an idea that you think is funny. Decide what it is. Make sure it's one that grabs you!
If you think it's funny then it must have a beginning and an end. Again, decide what that is.
What starts it off and what ends it to get a laugh in your mind? Somebody gets hurt, something happens , somebody embarrassed, what!?
Be prepared to do some creative verbal doctoring at this point and even stretching the truth to get the funny point across.