Combos are created by connecting smaller square pieces with larger ones near the bottom of the level, removing them completely. To do this, pop pieces above the similarly colored ones so they crashing down to remove them. You can either remove large formations or single squares to make the pieces drop down. Focus on the columns that are closest to touching the top of the screen first. At times you can remove just a single piece to trigger combos. Remove such tiles at the correct time to start longer combos. Work from the bottom and watch the left and right columns, as they seem to be the most difficult to control. Keep them as low as possible. Combos are more likely to remove tiles from the middle, allowing you to focus on the sides. Remove as many tiles as possible from the bottom to allow the top ones to create combos themselves.