Conventions of Composition Rule 106
Rule: Place an apostrophe before the final s to make a possessive singular of a work that doesn't already end in s. To make a word that already ends in s possessive, place an apostrophe after the final s. To make a plural word that doesn't end in s possessive, add an apostrophe and an s. Possessive pronouns (his, their, whose, etc.) do not need any apostrophes. Plural words do not use apostrophes. (For the rare exception of talking about the word itself, see rule 26.) Though you should usually avoid using contractions in formal writing, in more casual writing, you may use an apostrophe to replace missing letters.
Note: There are phrasal possessives (such as "the United Kingdom's crown jewels") and double possessives (such as "a friend of my uncle's"). Usually, you should avoid phrasal possessives and double possessives unless necessary for clarity. You might listen to this great discussion by Mignon Fogarty, Grammar Girl (episode 877). Start the podcast at 8:55.
Examples:
Wrong: The dog ate it’s bone.
Better: The dog ate its bone.
Wrong: The womens room is over there, but the mens is right here.
Better: The women’s room is over there, but the men’s is right here.
Wrong: The country’s flags were all hanging in a row.
Better: The countries’ flags were all hanging in a row.
Practice deciding where apostrophes should go in the following sentences:
While I dont know who's car's she had last week or earlier, I do know shes borrowing Haydens this week.
At Riley's and Jessie's party, people talked about what state's they're from.
Who do you think cobbled Jesuses sandal's?
You can find the womens' room slightly closer to the pool than the mens'.
Resources for further explanation of how to use apostrophes:
Your Dictionary's Apostrophe Rules
Grammar Book's Apostrophes