Go back down the stairs and into the common room. You have to backtrack a little again. Well, a lot actually, anyways, walk all the way down to the Great Hall.

Now, while you are walking down off of the giant steps, you will see two doors flanking the staircase. Go into the oneon the left to end up in the dungeon. Right when you walk through the door, you will be in between two more doors. Go into the one on the left to be in Snape's Potions classroom. Walk up until you have passed four cauldrons on the right side of the room, and you should see a cauldron stirring itself. Hermione is sitting right beside it. Walk past her until you see Professor Snape standing there. Talk to him, and he will tell you to take a seat

you go to the dongons on the main floor in the North west cornner and its the first door to you left.