I trained my 3-4 year old first the pressure release method, front back side and side, then in the round pen ..loose, and it was more like an arena...then worked to a lunge line...then introduced the bit, bridal, (copper half cheek snaffel) then rollar , then lunged with all the stuff (bridal and something around his middle) then ground drove , and he was a dream , and so light , then hooked him up to a small piece of wood to drag in the arena , then after he was totally OK with that , the cart with me walking behind it ...and he was PERFECT!!!! I had good luck with that method , and he has a lot of trust in me and NO issues.
good luck