As items are thrown randomly on the screen, use the mouse to pick two of the same kind and put them in the box, doing so to match all of the identical pairs of items, and when all of them have been placed inside the box, through matching them, the level is complete.
Do that before the time granted for each stage runs out on you, and know that the faster you move, the more money you are going to earn in return, for doing a good job. Each new level features more items to match and pack away, but more fun at the same time, of course. Good luck, enjoy!
Use the mouse.