One of the rules for teaching donkeys and mule to have Power Steering - GEE means turn right and HAW means turn left, enjoy this wonderful trainer and video.
Select this video to watch Cinnamon learn to pull a cart.
I was trying to learn the art of cart pulling before the semi truck accident and still pray one day I can ride again and train Rio and Rocket Man to pull a cart. I am sorry to say I can not help with cart pulling, Lucky Three ranch can and I believe Parelli has some training advise too.
GOD bless you and your family two and four-legged!