1. Most nouns form the possessive by adding the apostrophe and s ('s). Ex: man - man's; men - men’s, pupil - pupil’s, John - John’s.
2. Plural nouns ending in s form the possessive by adding only the apostrophe ('). Ex: persons - persons’, writers - writers'.
3. Some singular nouns ending in an s sound form the possessive by adding the apostrophe alone. Ex: for appearance' sake, for goodness' sake. But usage inclines to the adding of the apostrophe and s ('s) even if the singular noun does end in an s sound. Ex: Charles's book, Frances's dress, the mistress's dress.
4. When a compound noun, or a group of words treated as one name, is used to denote possession, the sign of the possessive is added to the last word only. Ex: Charles and John's mother (the mother of both Charles and John), Brown and Smith's store (the store of the firm Brown & Smith).
5. Where the succession of possessives is unpleasant or confusing, the substitution of a prepositional phrase should be made. Ex: the house of the mother of Charles's partner, instead of, Charles's partner's mother's house.
6. The sign of the possessive should be used with the word immediately preceding the word naming the thing possessed. Ex: Father and mother's house, Smith, the lawyer's, office, The Senator from Utah's seat.
7. Generally, nouns representing inanimate objects should not be used in the possessive case. It is better to say the hands of the clock than the clock's hands.
One should say somebody else's, not somebody else’s. The expression somebody else always occurs in the one form and in such cases the sign of the possessive should be added to the last word. Similarly: no one else's, everybody else's, etc.