Secure a partner to collaborate with you on your book. The best partnerships are between writers who complement each other rather than overlap in terms of knowledge and expertise. It's also important to consider the personalities involved. Meet the potential partner first and see if you think there will be a good personal fit.
Brainstorm the project carefully together. Develop ideas in partnership. Make notes individually and make sure you agree on things. Give your partner her say and ensure your ideas are heard and understood. If things are not going well at this stage, either seek a new partner or get help with communication skills so that you can proceed with the project.
Draw up a written agreement that includes payment details. Agree this before you begin writing.
Write and agree to a detailed outline of the work. If it is fiction, include as many plot points, characters and other details as you can. Lists agreed information resources and other inputs to the work.
Divide the work evenly between you. This may mean splitting the work chapter by chapter or some other way you're both comfortable with. For example, one might do the first draft, and the other the second. You might divide the writing in half. You might let your partner write descriptive passages, and you write the dialogue. Find a process that works for you.
Allocate the required research evenly in the case of non-fiction books. Or, you may allocate it according to your strengths. For example, your partner might prefer conducting interviews, and you might like library research.
Assign deadlines to your work. Meet your deadlines and ensure your partner keeps his. If he consistently misses deadlines, see if you need to revise the schedule. Keep the lines of communication open.
Edit your partner's work and allow her to edit yours. At this point you should have developed a strong relationship and understanding of your partner's writing style and aesthetics. Be open to changes. Give constructive criticism on her portion of the work and don't shy away from respectfully giving your point of view.