CPR is the artificial assumption of breathing and blood circulation when they arrest.
Especially the brains are most vulnerable: after four to six minutes without oxygen a (large) part becomes so damaged that normal function may no longer be possible. Fast action saves lifes. You learn it at every First Aid course, it is not difficult.

CPRCheck weather the victim is unconscious by appeal, shaking, shouting, pinching.
Look, listen and feel for breathing and heart rate.
In an unconscious victim the tongue may be sunken in the pharynx (troth). Open airway by tilting back the forehead and lifting the chin.
Give 30 chest compressions (approximately 18 seconds, or 100 per minute) successively on the chest and breath 2 times.
The rhythm of the CPR is appropriate in line with that of “Stayin ' Alive” by the BeeGees.

Keep the victim lying on the ground (not on a soft surface).
Kneel next to the victim. Bare chest. Place the heel of one hand on the center of the chest of the victim (2 fingers above the sternum).
Place the heel of your other hand on top of the first hand. Save the fingers of your hands together.
Press with arms stretched the sternum short and firm 30 times 4 to 5 cm deep.
The rate is about 100 times per minute. Counting aloud helps to keep the rhythm.

After 30 compressions open the airway once again. Pinch the soft part of the nose with the index finger and thumb of the hand that is on the forehead.
Take a normal breath and place your lips around the mouth of the victim. Avoid air leaks.

Blow steadily for about a second air into the mouth of the victim. Look at the movement of the chest.
Let the chest automatically return to its normal position and exhale. Do this 2x.

Keep these cycles: 30 x CPR, rescue breathing 2x. Repeat until the victim breathes or there is help.

When I had to use CPR in 2014 I got the advise to skip the breathing, and just to do CPR  (Latest instructions).