begin by looking at the mirror for too long. learn how to criticize every line and freckle you find. stare at your flaws and burn your imperfection into your memory until your sister is pounding on the door because you're taking too long. this is only preparing you for what you will face.
forgive adults for swearing in your presence. don't let your breath catch or your eyes widen. let these words take the place of the emotion you can't express. realize that this is how to feel something, at least for however long the word lasts.
forget how to smile. never show your teeth unless you are baring them. look around you and discover that there isn't much worth smiling about anymore.
the little things in life can only bring you so much joy. eventuallly, they will stop bringing joy at all.
let your friends and family change every part of you until you look into the mirror and realize this is what people want to see. let words get to you until they just don't any more. let glazed eyes and wandering thoughts be a defense mechanism rather than a product of idle time. convince yourself this is how it must be, forever and always until the rest of your life. there's not a lot left of it anyway.