Start with a broad sweep of the area, noting any scratches in a wall, scuff marks on the floor, or a notarized letter of confession signed by the perpetrator.
Wrap a perimeter of yellow barricade tape around all possible suspects.
Carefully label and photograph each individual piece of evidence immediately after you’ve tampered with it.
Wait a minute. What was that? Go back to that last photograph. Enhance that area. Yeah, right there. Can you zoom in a little? My god, that’s it—what we’ve been looking for this whole time! Right in front of our noses!
Any gawkers should be kept barred from the crime scene, unless of course they’re very curious.