How do you use the possessive to indicate a law firm owned by James?

There are two accepted forms for possessive singular nouns that end with an s:

Add an apostrophe (') after the existing s at the end of the Word:

James'

Add an apostrophe s ('s) after the existing s at the end of the Word:

James's

Examples:

James' law firm is in the Acme Building.

James's law firm is in the Acme Building.