There are already many good modding guides available. Have a look in the Scriptorium for example. However, if you are looking for a step-by-step guide, I am afraid I have to disappoint you. There is no such thing, and I doubt there ever will be. There isn't a set way to create a mod. There is just various things you can change. If you want to know how to change them, go to the Scriptorium or Modding questions.