Look up some videos for forward, backward and sideways breakfalls and shoulder rolls. When you can do these smoothly, progressing to doing them by jumping up then landing and going into the rolls.
Progress to dive rolls as well. Typically it's best to do diamond your hands and bend your elbows to absorb some of the force from a fall into the roll.
It's best to do the forward breakfalls (into a shoulder roll) on your forearms and not your wrists. Your wrists will be happy if you absorb it with the forearms. They won't otherwise.
For parkour, I train these first on grass or sand, then perhaps bare dirt then concrete/asphalt/blacktop. Learning these on a soft surface, like a gymnastics floor or judo floor or wrestling mats may teach you that the mats can absorb some of the force when in outside conditions they cannot.
For kids or out of shape adults, I would probably start with mats or some sort of tatami/gym floor.
Do not slap the ground. One, because where you do parkour is concrete and asphalt not tatami or canvas floors with foam padding underneath. Doing so would be a wonderful way to sprain your hand or possibly fracture your forearm if it were to hit something like a parking bump.
Tuck your chin on a backward breakfall. This will help so you don't whip the back of your head into the ground.