There are some basic rules to keep in mind when you're writing plural possessives, according to grammar expert June Casagrande.

A possessive is a word that shows possession or ownership of something. A possessive can be a noun, pronoun or adjective. Nouns are usually made possessive by adding an apostrophe and an ‘s ...

On Mavis Staples’s album, Dylan Thomas’ poetry, Xerxes’s armies, and other contested ways to make words ending in “S” possessive.