Instructions
Place hair into the cauldron that bubbles with Polyjuice Potion. Cast Wingardium Leviosa on Crabbe and Goyle, who sleep in the back right corner of the bathroom. They will drop their hair.
Put an apple into the Polyjuice Potion cauldron. Get an apple after you cast Wingardium Leviosa on the wheel over the toilet stalls and then jump across the floating toilet lids to the left to knock down debris. Build the debris and grab the apple.
Add a cherry to the Polyjuice Potion cauldron. Pick up the red wheel in the left side of the room and attach it to the hot water heater. Turn the handle and pick up the cherry that pops out with the stream of water.
Drink the Polyjuice Potion. Harry and Ron will turn into Crabbe and Goyle, aka Harry (Slytherin Disguise) and Ron (Slytherin Disguise). After you drink the potion, you will automatically appear in the dungeon, outside of the Slytherin common room.
Use the Lumos spell on the vines in the right corner of the lower floor. The vines will retreat. Behind the vines is a box. Use Wingardium Leviosa to open the box and then build a swinging pipe. This will cause four snakes to appear from sewer grates one after the other.
Destroy the four snakes. After each snake dies, it will turn into golden LEGO pieces. Assemble the pieces to build a moving knight statue. The statue will automatically jump into the pool of water and push the lever and stop the spurting water in front of the Slytherin common room entrance. Enter the common room.
Remove a fish from the aquarium in the right side of the room with Wingardium Leviosa. Follow the fish as it flops to the left of the room and use Wingardium Leviosa to lift the fish into an aquarium with a skull in it. Use Wingardium Leviosa to lift the skull out of the tank and use the spell again to remove the skull's eyes. The eyes will automatically go into a large snake's eyes and a door will open. Enter the door to end the level.