How?

How do I even start to form coherent thoughts from random racing? How do I get them down on paper? Some people say that if you know the end, write it out. The "how you get there" part will follow. Some say if you know the beginning, start there. The end will come. What if you know the middle part of a story in your mind, but have no idea how the characters got there or where they will go after the fact? Worse still, what if you have several ideas for the same story but have no idea how to form them together to make the story worth reading? Hmmmmm.......