Teaching a dog to turn in cart is a long process, and I urge you take your time with this.  Pushing a dog further than it is comfortable may cause the dog to lose confidence or spook in cart.  I start teaching turns by making large spirals in the cart.  Sometimes the radius of such spirals begin at 50 yards.  Once the dog is comfortable doing smaller circles (~20 ft in radius), I will start to teach a dog to turn and to perform side steps.  Once a dog can side step, I teach the dog how to do 360 turns.  Here’s two videos demonstrating how to teach a turn and how to navigate a 90 degree turn.

Teaching a dog to turn and perform 360s

Navigating a 90 degree turn