I filled the sink with warm water and Lux soap flakes, and Lou did the fun bit of rubbing the egg for a while and playing in the bubbles until the wool had shrunk around the plastic egg.
Here is what it looked like finished. While it was still wet, I cut a zig-zag half way across the middle, took out the plastic egg, re-shaped the felted egg then let it dry in the sun. On Thursday, Lou took it to school filled with little chocolate Easter eggs as a present for his teacher (which I forgot to take a picture of).
She loved it! :)