You take a feed sack(preferable cloth, or any kind of soft material). Rub it all over the horse, starting in the front. All the while talking softly, to reassure her, that it is not going to kill her. Work your way back. Then when she gets used to it, you can slap a little harder. You don't want to hurt or scare her at all.
I think the biggest thing about sacking out is that you don't stop doing whatever you're doing until they switch out of panic mode, realize that what you're doing isn't hurting them and relax and accept it. Then you stop. If you stop when they're freaking out, then you're rewarding them for freaking out.