Buy a current Writer's Market book. This is a valuable reference tool that you will go to repeatedly. Look for a publisher who is a good fit for your book. Request writers' guidelines from that publisher.
Read the guidelines carefully. Highlight important information such as how the publisher wants manuscripts sent. Some only want emailed submissions, others want postal mailed submissions, and some want both. Follow guidelines exactly.
Write a query letter. Make sure to include details about your book. Do not leave publishers guessing about the content of your book.
Include information about yourself in the query letter. Answer questions such as why you decided to write the book. Tell publishers any of your previous publishing history.
Double check that publisher's guidelines for submissions to make sure you are doing everything exactly as asked. Write another letter, thanking the editor for taking the time to read your book. Include any other information that the publisher asked for, especially any ideas or outlets you have to help sell your book. Read your manuscript one last time to check for typos. Sloppy manuscripts do not get a first look, let alone a second.
Send your manuscript to one publishing house at a time unless the publishers specifically say they accept simultaneous submissions.
Wait for a reply for the publisher, which may take up to a year. Contact the publisher only after allotted response time passes.
Take any specific reasons for rejection to heart and make changes to your manuscript if necessary. Send your manuscript to another publisher until your manuscript finds a publishing home.
An agent can also be the key to getting your book published. Follow the same guidelines for a query letter as you would to request an agent. If you look for an agent, find one that has the same goals and publishing ideals as you. Finding an agent takes time.
Self-publishing is another option, especially if your book is something that only friends, family or people in your area will be interested in. If you cannot get even one publisher interested in your book, do not automatically think about self-publishing your book. It may be that your book is not publishable at all. Take a second, third or fourth look at your manuscript. Pay for advice from a professional editor.