How should we indicate possession if a singular noun (or name) ends on s? Simply by adding an apostrophe and a possessive s in the usual way, e.g. the boss's Ferrari; Des's B&B; Tom Jones's diary.
Please note, however, that there are a few names ending on s that have traditionally been treated differently when it comes to indicating possession; in the case of these names we add only an apostrophe, instead of an apostrophe plus s according to standard practice. Some of these are names associated with the Bible, specifically Moses and Jesus, the possessive forms of which are Moses' and Jesus' respectively. I'll mention a few more next week.