Let's say you have years of social-media experience and have maintained a blog that has shared needed advice. You can put together your top blog posts and put together an e-book, such as The Ultimate Social-Media Guide for Small-Business Owners, and sell it on your website or Amazon. If you don't have any experience that you want to cash in on but have a knack for writing, say, sci-fi stories, then you can author and publish your novel on Amazon as well.
Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.