- Start at breakfast.
- Smile at him.
- When he smiles back, blink at him and look away.
- Smile at him everytime you walk by.
- Be at all of his Quidditch games.
- Cheer at him extra loud everytime He scores.
- After the game, tell him he played great.
- Let pass a few days just to be polite.
- Ask him if he could help you with your homeworks.
- After a few weeks, when you're good friends, ask him to go out in Hogsmeade.
- Tell him how you feel.

(But honestly, don't try this. It won't work if you aren't Lily Evans.)