This video picks up after the crocheter has finished the cube sides and shows you how to complete the decrease rounds to form the top of the cube. Don't forget to fill the cube before you complete all the decreases - there's a note about this on the video but ...
This video shows how to start making an amigurumi cube - rounds 1-3 are demonstrated.
What is amigurumi? According to wikipedia, it's "the Japanese art of knitting or crocheting small stuffed animals and anthropomorphic creatures." Pretty cool-looking stuff!