Read the project: Title, Instructions, Purpose, Play, Sprites, Events, Explore.

Read the code. Find the parts that control randomness. Find the parts that determine whether it's a face.

Cut out shapes and toss them randomly onto your desk. When do they look most like a face? 

Add a new sprite for a different face part. Follow the patterns you notice to make it random and check if it's good enough.

Create a more complex face from other shapes or emojis.

Try different shapes of different sizes; what is your best set of rules for "good faces?"