We've all felt (or will at some point) a one-way romantic connection. It hurts not to have your crush like you back, but if you're in the crush's shoes, there are a couple of things you can do to ensure that it goes as well as possible. Out of respect and empathy for the other person,
1) be direct, firm, and sincere -- tell them that you're sorry, but you don't feel the same way
2) be discreet -- make this a private conversation rather than allowing it to be gossip among friends; you don't want to shame them for expressing their admiration of you
3) handle the rejection quickly -- instead of letting their one-way crush for you go on and on (in the hope that maybe the problem will go away), face the situation and be done
4) don't ignore them or treat them differently afterward.
Think about how you would want to be treated.