There's absolutely nothing wrong with using other people's spell (or a book full of them), there are times when you may wish to use your own. It may be that you cant find what you're looking for or you might just feel a need to use original material. Whatever your reasons, its not hard.
(1) Figure out the goal or intent of the working. What you wish to accomplish? Are you looking for prosperity? Hoping to get a better job? Trying to bring love into your life? What is the specific aim of the spell? Whatever it may be, make sure you're clear on what you wanted.
(2) Determine what material components you'll need to achieve the goal. Will the working require herbs, candles, stones? Try to think outside of the box when you're composing a spell. (There's nothing wrong with using unusual ingredients in a working)
(3) Decide if timing is important. Some traditions, moon phase is crucial while in other its not significant. Remember: positive magic on waxing moon. Negative ot destructive magic on waning moon. If you're confident of not worrying about time with magic then go for it.
(4) Figure out what wording you want to use. Chant something formal and powerful to call upon something? Or mutter something poetic?
(5) Put it all together and there you have it.
What you need:
3: Your own Book of Shadows.