A back to back is when you follow a tetris (clearing four rows at once) with another tetris. An easy way to get this is to go to Effects Mode and play All Clear. In this mode you try to clear the entire screen in a limited number of drops. The rotation of puzzles seems to be random but you will get plenty of setups that have multiple back to backs. It should come naturally while playing. It took me maybe 30 mins, each round lasts about 1 minute.