I am an advocate of proper planning and preparation. If your library is quite small, you might get away with not creating a plan. But, if like The Authentication Library, your library is quite large and made up of more than a few files, then a plan is a really good idea.
You should create a list of the features that your library is going to have. This is important to do. Without a list of features, you could start developing the library and then keep adding more and more features as they come up. This is called scope creep and could kill your library before you even have a chance to release it. Having a list of features that you need before you start developing will help you stay on track, not allowing any more features to be added until you finish the ones that you have written down and release the library.
If you forget a few of the features that you want, leave them out. Release the features you had in the first place. If you add in the ones that you forgot...