You use a thin toothed comb and grab little sections or strands of hair and basically comb it the opposite way you normally would, towards your scalp. But, instead of combing the top of your hair comb from underneath. It helps if you use hairspray, too.
Don't just grab a handful of hair and do it though, it helps if you tease the top layers of hair, and then the layers under that and so on. I used a brush to tease mine though because I have really long and kind of thick hair, haha.
Good luck, and if I'm no help YOUTUBE. :D