Make it in one colour or as many as you can fit. Great for using up scrap yarn
https://www.youtube.com/watch?v=v2MMhBouLJc
Written pattern
http://www.bobwilson123.org/afghans--granny-squares.html
There is also a joining granny squares tutorial
https://www.youtube.com/watch?v=79O7UoCQXLo&feature=youtu.be