Write a cookbook that's new and different, with an appealing theme. For ideas, check out bookstores and browse the cookbooks section, trying to come up with a unique slant. Think about what kind of cookbook your friends and family would enjoy using...and would shell out money for.
Test your recipes thoroughly and be precise in your measurements and instructions. Consider supplementing your own recipes with recipes that are in the public domain and available to resell commercially. Make sure they ARE in the public domain, however, before you use them.
Type up your cookbook in Word or OpenOffice, including lots of appetizing pictures if possible. Lay out the cookbook with generous margins (at least 1") and format it with an easy-to-read font. Save the cookbook in .PDF form. You can either sell the book as an ebook in this electronic format or...
Self publish your cookbook in print. An ebook cookbook may be a reasonable option if you price it low, but most cooks will want a hard copy cookbook to refer to when cooking. So consider a print-on-demand service that will allow you to submit your book digitally and then prints it up for you and drop ships the book when an order comes in.
Use services that specialize in self publishing, either ebooks or printed books. Some of these services also offer marketing and distribution avenues, either the ability to sell ebooks online on their website store or to allow orders to be placed.
Market your self published cookbook in recipe forums, on your own Website, and by offering freebies to people who can give you testimonials. Take a signed copy of your cookbook to your local bookstores for the sales people to read, and offer to do a book signing. Tell everyone--family, friends, colleagues--about your cookbook.