Style Guide - Possessives

Singular nouns and names ending in s (or an s sound), a z or a silent s normally take an ’s. Chris’s sandwich, Heinz’s ketchup, Duplessis’s reign, Ross’s mistakes

Names ending with an iz sound usually take only an apostrophe. Moses’ commandments

Do not use an apostrophe if the modifier is more descriptive than possessive. girls rugby, boys residence

Memory tip: if the prepositions for or by would be more appropriate than the possessive of, do not use an apostrophe: rugby for boys, boys rugby; a guide for writers, a writers guide. Add ’s, however, when the plural does not end in s: men’s rugby, chldren's library

Exception: respect an organization’s individual style, even if it differs from these guidelines. Teachers’ Association

Its vs It's
Its is the possessive; it’s is a contraction for it is.
When tempted to use it’s, try the sentence with the contraction expanded. For example, The tree was losing it’s leaves, would become The tree was losing it is leaves. Sounds stupid, right? So switch to its.