In the marketing process, you will find that even among the publishers who publish how-to books, they may vary in how they want the book formatted. Some will accept books that simply narrate the appropriate advice; some will want numbered steps that take the reader through a process step-by-step; still others will require the addition of photographs or diagrams that actually illustrate each step.
One possible downside of writing and selling how-to books is that if you think you have spotted a particular interest or trend that would require a how-to book, chances are it is already too late to get a book written and on the market before that interest or need has already passed. You will find lists of book publishers interested in how-to books in Writer's Market and in the Christian Writers' Market Guide. For a list of how-to books specifically for writers, go to: http://www.stuartmarket.com.
For step-by-step help on writing a how-to book, go to: http://www.alpineguild.com/how-to_book.htm.