With great difficulty.  For a long time, even I didn't know what had happened in the back story.  I had to rewrite and keep on rewriting, and in the end the book became completely different.   I have worked on two murder mysteries/psychological thrillers. I scribble down the mechanics of the plot and refer to the paper when I'm writing.

 *Simpler, the better

 * Intuitive grasp of events - a feeling that a certain character might behave in a particular way (or an adult may remember specific events that are unique to their personality)

 *Some (basic) knowledge of police procedures and forensics, although I tend to leave it out

*Emphasis on how the crime effects the community

 *The use of viewpoint to place varying angles on a set of events