As it says. Pick a puzzle (the kid's pack is the easiest and the cheapest collection) with 48 pieces on Expert.
Place the side pieces in the box (there is an option where you can rearange the pieces) and start making the puzzle's inner part. When done, attach the sides to the puzzle. Achievement should pop once the puzzle is done at the summary screen. If you did not get it yet, the achievement for finishing a puzzle on expert in less than 45 minutes should pop too.