Find the crest you want to knock down. Cast Wingardium Leviosa on the object you want to use to knock it down. Note: This object can be anywhere and does not have to be in throwing range of the crest. Using either a Dung-bomb or an Exploding Cauldron is recommended. Once the object is in the air, hold [Run]. You can now walk around the castle and grounds while still carrying the object in the air. Make sure that you keep [Run] held until you get to where the crest is located. Note: During this time Harry will not run fast. Throw that object at the crest to knock it down.