To perform chest compressions, the rescuer places the heel of one hand in the center of the chest over the lower (caudad) portion of the sternum and the heel of their other hand atop the first. The rescuer's own chest should be directly above their hands. This enables the rescuer to use their body weight to compress the patient's chest, rather than just the muscles of their arms, which can fatigue quickly.
See UpToDate topics for further details about basic life support for adults.