1. Do some market research. Find out what is selling now. Look for a book--or better, a series--that has a large, dedicated--preferably a little fanatical--fanbase.
2. Write an Alternate Universe (AU) fanfiction for the world you chose. (If you've never written fanfiction, that's okay. There are lots of examples on the interwebz. Just Google it. Besides, fanfiction doesn't have to be good, as long as it's set in a world with a large, obsessed fanbase.)
3. Change the names of the main characters. See, the problem with fanfiction is you can't sell it because the world you're working with is copyrighted; it belongs to the original author. However, if you change the names of your characters, and you're writing an AU, that makes it okay. (Also, make sure if one of the original characters was a vampire, make him into a regular human but a misogynistic, self-centered jerk who likes to hurt women and control people.)
4. Include lots of sex. The more the better. It doesn't have to be well-written, though, so don't worry about that.
5. While marketing your magnum opus, let it slip that it started out as a fanfic and mention the book/series you copied from; this will ensure that all or most of the fans of that series will read your work.
6. Sit back and watch the royalties come in.