Define your scope. Leadership is an extremely broad topic, so defining what area of leadership you are discussing is crucial. Questions to consider: Who is the audience? (CEOs, Military men, housewives, teachers, etc.) What is the basis? (Biblical, scientific studies, personal experience, etc.)
Outline your book. Arrange your book from beginning to end. Keep in mind, readers prefer logical progressions, such as chronological (based on time) or topical (each topic building on the previous).
Research the competition. In order to avoid saying the exact same thing someone has already said, you need to research the competition and find out if what you are offering is a unique viewpoint. If it is not, how different is your view from another author's and how can you differentiate it better?
Write your first draft. The only way to truly write a book is to simply write.
Proofread your first draft for content. You want to make sure you have included all relevant information. This is also a good place to allow someone else to read it. Have them write down any questions your first draft brings up, then incorporate the information necessary to answer those questions into your next draft.
Write your second draft. This draft should be complete content-wise.
Edit your work for technical issues. Grammar, syntax, word choice, spelling and consistency of authorial voice are all technical issues you will need to address.