Here's how it works.
First, log into kdp.amazon.com to look at the Kindle books you have in their system. You'll be advertising one of them.
To the right of your book, you'll see a "..." (more) menu option. Click that. Click the advertise option in there.
Click that yes, you want to run an ad for this book.
Now you're in Amazon's advertising interface. You're asked if you want to stick your book in a results listing or if you want to have your book to show up immediately with other related books, on those books' pages. I highly recommend the second. Your far best chances of success is if someone is on a book just like yours and then sees your book. In comparison, if they're just randomly browsing for "mysteries" your book could be wildly different from what they like to read. The better you can target, the more sales you'll get.
You next get to choose either by related specific books or by generic interests. Again, generic interests just don't get you where you need to be. Go with the specific book matches. You don't just want people who read "mysteries." You want people who read "gentle relaxing mysteries with a heroine who is middle aged who likes nature." You can get that specific by knowing your genre and knowing who else is out there.
Now it's time for the settings. Choose a price CPC (cost per click) that is in line with current averages, so your ad gets run. You want somewhere in the middle of the range. For the budget, at the time of this writing the minimum budget was $100.
Now it's time to write your ad copy. MAKE SURE IT IS TYPO FREE!! I've seen ads with typos and there is no way I'd ever click on one of those. You're throwing money out the window. Make sure it's alluring. Don't just cut and paste the top of your description. Optimize for this small space you have. It has to grab the reader and make them think "I like this kind of book." Hopefully your cover already does that at the tiny size it'll be shown at. If not, you need a new cover, because thumbnail-power is key for covers in our modern age.
Amazon will now summarize all of your settings and send the ad off to be reviewed. They won't just run it because otherwise porn people could put porn ads in the children's book section :).
Let me know your results with Amazon ads!
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