This should come naturally throughout play and links in to both the "Child's Play!" and the achievement that follows "Elementary!", "My Grey Matters".
If you've earned the first achievement, which asks you to complete 25 of the puzzles, you should have no trouble getting this. Just continue what you were doing for that achievement until it (and "My Grey Matters" unlocks). As long as you have those three achievements then you don't need to do the puzzles, as many of the late game ones are rather difficult.
For those who haven't started the game yet, here's the link to the TA walkthrough, which will walk you through what counts as a puzzle or mini game and what doesn't:
Sherlock Holmes: Crimes & Punishments Walkthrough