You have to make that the required amount of yetis enters the house before the time is up! With 20 levels of increasing difficulty.
Keep pressed the left mouse button on the yeti, then move the mouse where you want the yeti to go, and release it to make him walk. To stop the yeti, click him. You can go faster pressing the speed button ("> >") or keeping pressed the F key. To move in a given level to see the full scene, use the arrow keys or the W-A-S-D keys. You can also move the mouse near to the borders of the screen.