This might not make much sense, but which one of these is the correct one to use as a title? - The Good Girl's Guide to Being Bad or - The Good Girls' Guide to Being Bad (It's about one person, I'm just really bad with grammar and unsure)

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Since it’s about one good girl, it’s The Good Girl’s Guide to Being Bad.

Basically The Good Girl’s Guide to Being Bad would be for any story that chronicles one “good girl”’s actions or a guide from one person calling themselves a good girl/the Good Girl.

The Good Girls’ Guide to Being Bad would be either if it was a story about several girls (think either a group’s story like Pretty Little Liars or intersecting or parallel stories like in Love Actually or Babel) or it’s a guide addressed to all “good girls”.

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