One way to learn how to slap is to roll freely. You don’t create a form and hope that it will match the ground or the body. You are aware and free to move to match the ground. The same goes for open hand strikes (from any angle front side and back) for example start by loosening your wrists and shaking the hands so you feel the blood flow in them and the pulse in the forearm. Now slowly make contact using your wrist and let the palm and fingers wrap around the flesh or anything you are using. Move slowly and do not forcefully apply pressure. This will create tension in the hand and you will get the force of the slap back. With a partner or on yourself practice this and as you feel comfortable speed up and down to match the level of force you want to generate. A slap can take the wind of someone without doing too much hdamage (sides where the lung muscles are, lower back of the neck, the small of the back and so on) and more importantly it is not considered an attack on the level of a fist to most audiences. Remember where you are at all times and make your choices.