If the word is a proper noun ending in “S”, you add an appostrophe and another “S”. For instance, the hat of Bob Jones becomes Bob Jones’s hat. The poetry of John Keats becomes Keats’s poetry.

If the word is a common noun ending in “s”, purists would add only an apostrophe. So the tiara of the princess would be the princess’ tiara. However, contemporary usage allows the addition of another “s”, so it would be princess’s tiara. This sounds more correct to the ear too. Ultimately, the important thing with common nouns is to pick one style and stick with it consistently.