Put on your goggles and work gloves. There will be concrete chips flying as you break up the concrete, so you must take pains to protect yourself.
Walk to the edge of the concrete and dig under the slab a short distance. This will remove the ground support from beneath the concrete and allow your sledgehammer swings to be more effective.
Grip the sledgehammer with both hands and stand with your feet about shoulder-width apart. Take a couple test swings so you are comfortable with your grip and stance before actually hitting the concrete.
Insert a crowbar, pick or mattock under the dug-out area of the concrete and have a helper lever the concrete up slightly.
Strike the concrete face about 6 inches past where your helper is levering. Continue striking until the concrete cracks and separates from the rest of the slab.
Repeat these steps as needed until the concrete is broken up.