Form the possessive singular of nouns by adding’s.
Follow this rule whatever the final consonant. Thus write,
• Charles’s friend • Burn’s poems • the witch’s malice
Exceptions are the possessives of ancient proper names ending in -es and –is, the possessive Jesus’, and such forms as for conscience ‘ sake, for righteousness’ sake. But such forms as Moses’ Laws, Isis’ temple are commonly replaced by
• the laws of Moses • the temple of Isis
The pronominal possessives hers, its, theirs, yours, and ours have no apostrophe. Indefinite pronouns, however, use the apostrophe to show possession.
• one’s rights • somebody else’s umbrella
A common error is to write it’s for its, or vice versa. The first is a contraction, meaning “it is.” The second is a possessive.
• It’s a wise dog that scratches its own fleas.
William Strunk and EB White