When you walk into the building itself, you can go left or right. Right is the lockers. Left is the line. Go to the left, where the line is... but instead of veering to the left and continuing straight with the rest of the line, it's actually a sharp left turn.
It can be hard to see, it's dark in there and you've just come in from the light -- so just ask if you don't see it. There is someone who usually stands near the actual line, if you've gone past that person, you've gone too far. And if you've made the turn with the regular line further into Hogwarts, you've gone too far.