The Elements of Style · Rule 1

Form the possessive singular of nouns by adding 's.

Follow this rule whatever the final consonant. Thus write,

Charles's friend
Burns's poems
the witch's malice

This is the usage of the United States Government Printing Office and of the Oxford University Press.

Exceptions are the possessive of ancient proper names in -es and -is, the possessive Jesus', and such forms as for conscience' sake, for righteousness' sake. But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced by

the heel of Achilles
the laws of Moses
the temple of Isis

The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.

In plain terms

To show that something belongs to one person or thing, add an apostrophe and s to the noun. The pattern holds even when the noun already ends in s or an s sound, so you write the bus's route and Silas's notebook. One simple pattern saves the reader from guessing, and it matches the way most people say these words aloud. Strunk allowed only a few traditional exceptions, mostly ancient names.

BeforeThe boss' decision surprised everyone, and Silas' reaction surprised them even more.

AfterThe boss's decision surprised everyone, and Silas's reaction surprised them even more.

Today: Most guides agree for common nouns: the boss's office, the class's project. They split on names ending in s. Chicago and MLA write Silas's, while AP news style writes Silas'. Whichever you choose, use it consistently. Plural nouns ending in s still take only an apostrophe, as in the players' bench.