Once you self-publish a book on Amazon, you can offer a free book promotion of the Kindle edition. By default, it seems we are allowed 5 such days of free Kindle promotions for a book. The screen is pretty straight forward, and here is a screenshot of part of the screen I used to set my upcoming promotion on Monday, July 13.
Once you select the Free Book Promotion option, you simply add the date range you want the offer to be in effect.
Once that is set up, promote it on your social media! Here is how I am doing my boot strap promotion:
I admit that I probably have more eyeballs available on Facebook, so here is what the promotion looks like on my Facebook page: https://www.facebook.com/midlife.awakening.book