Backcombing is quite simple. Grab the hair that you want to backcomb. With a comb (I use flea combs because the metal bristles are stronger than plastic) start brushing backwards, starting at the root and working your way down to the ends. DONT start at the end of the hair and brush up. You dont want the ends to be combed up to the roots. This will create a total mess of chunky dreads. Keep the strokes short. After you have done this, palm roll and repeat until the dread is tight.
Note: backcombing is great for fat dreadlocks, but will result in the loss of length. If you are looking to keep your length, refer to my post on how to crochet. :)