Cardiopulmonary resuscitation is an emergency procedure that combines chest compressions often with artificial ventilation in an effort to manually preserve intact brain function until further measures are taken to restore spontaneous blood circulation and breathing in a person who is in cardiac arrest.
Anyone can do the CPR, may they be untrained, trained and ready to go, or trained but rusty. In doing the procedure, you should put the person on his or her back on a firm surface, kneel next to the person’s neck and shoulder, place the heel of one hand over the center of the person’s chest, between the nipples, place your other hand on top of the first hand, keep your elbows straight and position your shoulders directly above your hands. Use your upper body weight as you push straight down on the chest at least 2 inches but not greater that 2.4 inches. Push hard at a rate of 100 to 120 compressions a minute. If you haven’t been trained in CPR, continue chest compressions until there are signs of movement or until emergency medical personnel take over. If you have been trained in CPR, go on to opening the airway and rescue breathing. Doing the CPR on an adult can be different on a child or a baby, most cardiac arrests in babies occur from lack of oxygen, such as from drowning or choking. If you know the baby has an airway obstruction, perform first aid for choking. If you don’t know why the baby isn’t breathing, perform CPR.
CPR alone is unlikely to restart the heart. Its main purpose is to restore partial flow of oxygenated blood to the brain and heart. The objective is to delay tissue death and to extend the brief window of opportunity for a successful resuscitation without permanent brain damage. 
https://www.mayoclinic.org/first-aid/first-aid-cpr/basics/art-20056600
https://en.wikipedia.org/wiki/Cardiopulmonary_resuscitation