While the TV series leaves everything intentionally vague, the book gives a pretty in-depth explanation of how everything went down. The story stretches a bit farther — Amma is incarcerated and charged for the murders of Wind Gap's young girls. When Camille stops in for a prison visit, that's when Amma fills in a lot of the blanks.
The novel equips Amma with three blond friends who help her commit the murders. This trio would hold down the victims while Amma strangled them. We only get small flashes of these murders in the TV show, and they come during an end credits scene.
Amma's motive is also much clearer in the novel. In terms of Adora's Munchausen syndrome by proxy, Amma seems to enjoy it almost as much as her mother. For Adora, it's a need to be needed. For Amma, it's attention that she craves. Amma confesses in the novel that Adora had grown affectionate toward her friends, and also that her friends had begun to ask questions about the frequency of her illness. She killed them so that they wouldn't "ruin" the relationship between her and Adora.