Grab a block from anywhere under the last completed row and then put it on the top - that's it! The longer you play the harder it'll be as the tower becomes more and more unstable, however, you'll also unlock stuff faster. Good luck!
GUIDES
FAQ
Stuck and need some help? Check out some of the questions the community have asked relating to this game. If none of these answer your problem or you want to get some tips when it comes to a particular level, feel free to submit a question of your own.