User blog comment:SibunaSeason34/February Vent Blog/@comment-24596535-20150219191001/@comment-4816103-20150219191517

One way you could do a mystery is by figuring out what the ending is, you know, what the whole big secret was (maybe even write the ending first) and then go back to have all the foreshadowing and clues and redherrings because, well, you already know where the story will end up. That could work with any mystery, really. Just think of an unexpected outcome (that, well, actually makes sense with the story), and then work to build it up to that.

In other words, work backwards.