It’s easy. Or it should be. But plural possessives get confusing because the letter S has too many jobs in English and they all get jumbled in our heads. In English, S is used to form plurals.

Columnist and author June Casagrande ponders singular vs. plural nouns, plus the nagging questions behind subject-verb ...

Plural nouns refer to more than one person or thing, for example 'brothers'. To show possession when the noun is plural and already ends in 's', you just add an apostrophe to the end of the word ...

Plural nouns refer to more than one person or thing, for example 'brothers'. To show possession when the noun is plural and already ends in 's', you just add an apostrophe to the end of the word ...