start small, punch or kick yourself, really builds up the muscle, then punch wood, like bruce lee did tense up at last moment, then move to steel/metal, concrete, then stone

WITH FEET, you must train your heel, then sides of foot, then gently train your toes by using them to strike the aforementioned surfaces above, be gentle or you’ll lose a foot

WARNING: THERE IS A CHANCE YOU WILL BREAK YOUR LIMBS