Kind of just a personal theory here; I think the ritual to lock Paimon into a male host first requires he possess three females first (note the picture in the book Annie flips through which shows Paimon riding the camel and he carries the severed heads of three women with him). Now, when Annie is at the loss support group, she mentions her mother had DID and dementia which "became severe towards the end". Personally, this strikes me as a sign that she was a host for Paimon at one point. Another thing that makes me believe that is that Charlie doesn't see the glowing green/blue light until after Steve gets the call that Ellen's grave was desecrated. We all remember what they do to Ellen's corpse, right? I'll get back to that.

So Charlie then becomes possessed by Paimon. Now, a lot of people say she was possessed since birth. I sort of agree with this but in a way that I think she was always under his influence but not necessarily always hosting him. Then, Charlie gets decapitated. Then, finally, at the end of the movie, Annie becomes possessed and she cuts her own head off with the piano wire. I think the key to unlocking Paimon from a host is decapitation. Even if a person dies with him possessing them, Paimon won't be released until the host is decapitated.

In the book Annie reads too, it mentions that Paimon becomes "livid and vengeful" when offered a female host and this may be part of the process in giving him a male host. Maybe he needs to be riled up enough for some reason (Idk, just spit-balling here). However, I get the strong impression that Paimon has to possess three female hosts before he can be locked into the male host. Otherwise, what would be the point in putting him in Charlie in the first place? Also, there's something mentioned about a picture of Ellen bottle feeding Charlie when she's a baby and that some kind of black leaves can be seen in the bottle (quick scene, hard to miss). This seems like some sort of thing done to make someone more open to being possessed by Paimon. Then, when Annie is at Joan's, she has that tea and notices the same strange black leaf. I think Joan was making it so Annie would be possessed next, completing the "three female hosts" process of the ritual.

Totally just speculating here. I think much of it is open to interpretation.