After you have called for emergency services and deemed the situation safe in order to approach the individual, the American Heart Association (AHA) advises taking no more than 10 seconds to check for signs of breathing or a pulse (via UCDavisHealth). As you proceed, follow the 30-2 rule, which refers to 30 chest compressions followed by two breaths.
To do so, first ensure the person is positioned flat on their back and remove any clothing that may hinder CPR, if possible. With one hand, place the heel of your palm on the bottom half of the breastbone, while placing the heel of your opposite hand over your initial hand. Compress downward approximately 2 inches and maintain a rhythm of 100 to 120 compressions every 60 seconds. After every 30 compressions, tilt the individual’s head back, lift the chin, and administer two breaths for a duration of 1 second each. Continue this cycle until emergency personnel are on the scene or the person becomes responsive. Note that if two people are able to administer CPR, they should switch out every 2 minutes.