What is one of the rules for forming possessive case words that end with an S or s sound?

One rule for forming possessive case words that end with an S or s sound is:

Add an apostrophe only (no additional "s")

For example:

* James's book (incorrect) becomes James' book

* Boss's office (incorrect) becomes Boss' office

This applies to both singular and plural nouns ending in s or an s sound.