The possesion process is very simple, many people don't know much about it but it's very interesting. It's just a matter of the underworld taking the life of an object or even a person in the real world. One of the most common stories/movies of a possesion is the movie, "The Excorsit." But sometimes, people posses dolls by doing a voodoo ritual. The most common movie relating to that is, "Child's play." The voodoo ritual is not so common, people usually do these rituals to keep spirits away or just to have fun with it
Annabelle was possesing the Raggedy Anne doll, but she didn't do harm to Donna nor Angie. She wanted to hurt everyone. You can see some stories in our story collections all about Annabelle. Annabelle simply just put her spirit into the doll. No one knows wether Annabelle was possesed before Donna's mom bought her it, or if she became possesed while staying with Donna and Angie.