This is done as a multiple step process and will be done over several classes.  A five lesson plan is shown below. Each student will create their own version of this game with their own character and background, and their own rules for movement/food/enemy. There are  enough variations to keep the game interesting and different from any other game in the class.

More details: http://www.digitalartforall.com/790/scratch-making-a-simple-game/