How to make singular and plural words that end with "s" possessive. It's an apostrophe style choice.
What About Plural Words?
I always feel bad when the answer is that there isn't an answer, so here's an easier situation that has a firm rule: if the word ending with s is plural, such as aardvarks, then you just add an apostrophe at the end to make it possessive. For example, you could write, "The aardvarks' escape route [s apostroph was blocked" to indicate that a family of aardvarks needed to find another way out of danger.
Plural words that don't end with s, such as children, do take an apostrophe s at the end for possession. For example, you could write, "Fortunately, the children's room [children apostrophe s] had a hidden doorway."
Here's a tricky issue with a definite answer: how do you make the plural of a single letter, as in Mind your p's and q's? It's shocking, but you actually use the apostrophe before the s! It looks possessive, but it isn't. The apostrophe is just there to make it clear that you're writing about multiple p's and q's. The apostrophe is especially important when you are writing about a's, i's, and u's because without the apostrophe readers could easily think you are writing the words as, is, and us.
Next: Should You Use Apostrophes with Abbreviations?