Do your cousin or does your cousin?

Could you please clarify the proper usage of "do your cousin" or "does your cousin" in a sentence? I'm curious about the grammatical rule behind choosing the correct form of the verb "do" when referring to a third-person singular subject like "your cousin." Is it always necessary to use "does" in such cases, or are there any exceptions to this rule? Understanding this distinction would help me avoid making similar mistakes in my own writing.