When talking about possessives and the rules of writing apostrophes, we need to take into account several scenarios:

1. Adding ‘s to the singular form of the word (even if it ends in -s):

the neighbor’s dog
James’s moustache (or we can write James’ moustache, depending on the Style Guide observed and/or text consistency).

2. Adding ‘s to the plural forms that do not end in -s:

the women’s play
the children’s toys.

3. Adding ‘ to the end of plural nouns that end in -s:

three cats’ sandbox
ten friends’ group expenses.

4. Adding ‘s to the end of compound words:

your mother-in-law’s birthday.

5. Adding ‘s to the last noun to show joint possession of an object:

Amy and Mark’s trip.