Write and publish an eBook. You don’t need a traditional publisher and financial backing to publish your own book anymore. In fact, Amazon.com makes it possible for you to publish your own eBook and sell it independently – with no financial investment on your part. And with Kindle eBook publishing, your book will appear on Kindle stores worldwide within 24-48 hours. Just be prepared to market it yourself on social media, your blog, or elsewhere if you expect to generate sales.
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.