@ZebulaJams
It’s important that you understand that expression pedals only control parameters inside the Axe Fx, and have no wah sound of their own. Without this firmly in your head, it might be harder to grasp the great instructions provided by
@chris and
@Moke above. Just so you know, those two guys are among the very best at using these functions and pedals, and at explaining those uses to others, so you are in good hands.
Just think of the expression pedals as a remote hand turning a “knob” inside your unit, raising and lowering that control as you move the pedal. You assign what it’s controlling by assigning it in the Axe Fx. Auto-engage, a very powerful function, can be used at the same time so that a certain portion on one end of the pedal travel also turns the effect on or off.