Firstly you're going to need to unlock Snape.
Snape can be found in the secret room in Potions classroom located in Hogwarts.
In this room there will be 3 task to complete that require certain characters, they are the following:
A painting that wants to see a member of Hufflepuff - Justin Finch-Fletchy (Unlocked in Year 2)
A puzzle to create a goblin vault - Griphook, the goblin with the key (Unlocked in the first level in Year 1)
A bookcase - Harmione (She is always with you)
Doing those 3 tasks will give you 3 keys, use those keys to release the puzzle blocks to build a platform up, the Snape unlock is to the Left of you in a chest. (Note: There is a Red Brick to your right as well)
Now that Snape is unlocked you need to head to Diagon Alley and go to Madam Malkin's Robes to purchase Snape (He costs 250,000studs, i think).
Now that you have Snape purchased start up the mission "The First Task" which is from Year 4 and there is a barrel directly at the start OR if you arent very far in to the game you can do the first mission and once you leave The Leaky Cauldron there is a barrel on your right.
Note: Snape MUST be assigned to Player 1.