How?
Whenever I've tried, it comes back with exactly zero people. You try to activate it, and it won't.
The options are there to target your group or page, but they don't work. Or never have for me.
There are two types of ads: Boosted posts where you create a post on your timeline, and then boost either to an audience you create or to your fans and their friends. These can be text only, or in a vertical format, or just basically ANYTHING that doesn't violate FB's rules.
The other type are targeted ads. These can be videos, slideshows, or single pictures. You create an audience by selecting their interests and narrowing their habits. For instance, you might pick people who like Robert Heinlein who also own or are interested in pages relating to Amazon Kindle.
Both types of ads work for me.
I use boosted posts for new releases. I create a post, share it, and when it stops getting clicks I boost it to all my fans and their friends. The nice thing about boosted posts is you can have more than one link. So I'll usually say, "Get the new release <link>." and then, "Haven't started the series yet? The first book is FREE <link>"
I boost them to fans and their friends because my fans, when they see the boosted post, usually comment on the post saying lovely things like "I love this series" or "I'm reading it now and it's so exciting." That gives you social proof.
I also run targeted ad campaigns mostly for the US and UK, although I am experimenting with worldwide iBooks ads right now. I am sending everyone directly to the vendor of choice.
Facebook ads are more expensive, but I find that FB viewers are more likely to buy paperbacks and to buy audiobooks. Also, since they aren't LOOKING to buy books on FB, if they click through, they tend to be very motivated. I'm advertising a permafree, but the sell-thru from freebie to first paid book with FB is the best of any advertiser.
I learned to use Facebook by picking up a book from my library. Also, I regularly Google things like "Facebook advertising in year 2020" to catch up with current trends. I tend not to follow authors teaching authors because I feel like I'll just wind up doing what every other author is doing. I have found that general advice works well for me.