All you have to do is remember that if there’s ownership or possession, then the word should take apostrophe -s. If there are many (the word is plural), then just an “s” will do. If a word is both plural and possessed, it gets an s followed by an apostrophe. And for the word “it,” the rules are reversed.

The same rules apply to plural nouns to help show possession. Use an apostrophe + -s after a plural noun. But, only use an apostrophe after the -s at the end of a plural noun that ends in an -s. The additional -s are never needed with plural possession.