The best way to back comb hair is to take a small section of hair about 4cm by 4cm, once you have the section holding it tight, using a dressing out comb or if you do not have one of those you can use fine tooth comb.
Put the comb in to the middle of the section then start using backwards and forwards motions making sure that when you go back towards the head that you push the backcombed hair down this will make it tighter.