It just means write within the genre, take a subgenre such as the puzzle school or the harb-boiled school and write a story using those conventions. I.e. (this is a crap example but anyway) a story set within a country manor where one of the suspects of a crime is inside the house, the weather reflects the mood, etc or have a detective 'loner' that tries to fight crime in the big-bad-word like Marlowe in the Big Sleep. Oh btw, don't just write using the conventions of the genre, feel fre to suvert and challenge notions of the genre like Ondaatje does in Anil's Ghost.
Anyway, my suggestion is to have a murder, they always creat 'mystery' which is usually (but not always) the focus of the text...
Anyway, i shall stop rambling. Hope that helped a bit