If you are CPR certified and the victim is not breathing, the American Heart Association recommends combining chest compressions with rescue breathing. After performing 30 chest compressions, the rescuer will clear the victim's airway by tilting the head back and the chin upwards.
Next, they will place their mouth over the victim's mouth so it makes a seal and delivers one rescue breath, checking to see if the victim's chest rises. If the chest rises, the rescuer can resume chest compressions. If the chest does not rise, they can deliver another rescue breath and then return to compressions, repeating the sequence of 2 rescue breaths for every 30 compressions.
If two rescuers are present, one will perform rescue breathing, and the other will perform chest compressions. Rescuers should switch positions every couple of minutes to avoid fatigue.