Step-by-step explanation:
You can't. I know this probably isn't what you want to hear, but this is their fight. You can help them, reassuring them that they look great!
Tell a trusted adult.
I know it might feel like you are betraying them, but this is what you have to do. Anorexia is a very serious mental condition and shouldn't be taken lightly. It can even result in death.
Love them and care for them, don't treat them like a baby, treat them like just another human being. Never bring up their looks unless in a positive way. However, don't do that too much, as that can make them rely on you to feel good.