I find this to be easiest at the beginning of the game to set up your screen so that you have 2 open columns on the left or right, you're going to need to build it pretty high. Then simply drop pieces down the gap focusing on making single and double line clears. These are consecutive so you can't drop random pieces onto your pile in between line clears, but you can use the hold piece option.
After 8 or 9 line clears in a row it should unlock. There will be a bubble that pops up telling you how many combos you are up to. Dumbledors Army has a great video with this as the first part:
http://www.youtube.com/watch?v=cF1a6H06Nek
