Go to Snowman's Land and climb through the area. Now, get to where the large snowman is blowing at you and the penguin is walking around. Wait for him to get near you, and then backflip onto him. You're now riding on top of him, and sort of through him. Now, if you inch off his head a little bit, you'll fall but catch yourself, and hang over him (like when you almost fall off an edge and hang off of it). You will be in the penguin's head, with only your hat poking out as a signal to the Mario inside.
Submitted by: Connie