Go to Koopa Troopa Cave (W.1-2). Eventually, you'll find an area with a Koopa and a small, U-shaped tunnel (this is all underground). You have to face the wall (be in the smallest part of the "U" (one of the "handles")) and throw a shell at it WHILE jumping right after (go in big, as you might get hit). If done correctly, your character should keep bouncing on the shell, leading to the...