Choose your religious topic, whether it is a particular religion or an experience within a religion.
Organize your schedule so you have a specific time period to write. This may be first thing in the morning, the time you spend on the subway or after dinner. If you don't organize your time, you may find yourself with no time to write.
Research the topic. You need to know everything there is to know about this religion, as well as its impact on society and culture. It is important to understand not just the basic tenets of the religion, but also the historical and current significance of the underlying philosophy espoused within this tradition.
Decide how you will tell the story or share the information. You might choose to write an academic book about a particular religion, or you may decide to make your book more personal. You can share new information you learned from practicing this religion, or you can create a memoir. Knowing how you want to present your information is the first step to a successful book.
Write an outline. This will help you organize your thoughts and ideas and give you a sense of where you are going as you write the book. Sitting down and writing without an outline can be very confusing and will create more work than necessary. An outline will keep you on track and help when it is time to write the book proposal.
Decide on traditional or non-traditional publisher. Traditional publishers require literary agents, while non-traditional publishers are considered self publishing.
Write a non-fiction book proposal if you want to use a traditional publisher. For this avenue, you do not need to write the book first. The proposal is used to secure a literary agent. The book is written after the proposal is accepted. Each agent has her own list of requirements for what she wants to see in a proposal, so it is important to create a target list of agents and research what they want before sending anything to them.
Write the book. This will have to be done first if you are using a non-traditional publisher, but either way you have to do it.
Ask friends, relatives and other writers to critique your work before sending it to literary agents or into the world to be published. It is easy to be too close to your writing to pick up on basic mistakes, and therefore, it's important to have others offer input.
Hire an editor to do a final edit. If you are working with an agent, he will help you with this process. If you are self publishing, this is a critical component of writing the book.