Instructions
Play through the Hogsmeade level in Free Play mode until you melt the large snowman at the top of a hill. You'll see this snowman directly to the north after exiting the Honeydukes shop.
Go to the left from where you melted the snowman and switch to a character with a digging ability like Fang, Hagrid's dog. Dig on the shimmering pile, and a giant snowball will appear along with three bowling pins at the bottom of the hill.
Ride the snowball down the hill and strike all three bowling pins. You can control the direction of your descent using the "Move" control for your particular platform, such as the left analog stick on the PS3 or Xbox 360 controller. After you hit all three pins the Gryffindor Crest will appear, so grab it and continue on in the level.