Tips on Neck Reining?

My Paint is an ex- barrel horse thats now just basically a pasture pet, but i still work with her and ride. Shes overtly sensitve with the bit because of my dad, but she has settled a bit since ive started working with her. But im not too good at neck reining, and she is a little rusty too... Any tips on polishing it up, and more info on how to do it?