There is already a great answer from conejote with all the basics. But, what if the plastic piece that has the teeth on it is broken. He said make sure that it isn't broken. If it is, do you have to take the whole mechanism out and retrieve the piece? Does the piece fall out where it won't cause a problem? I have the old one out, and the new one purchased. I am not sure if I can put the new one in with out damaging something. Otherwise all of conejote's instructions are great.