CPR is one of the most basic and important emergency medicine skills to have in your bag of tricks, as knowing how to properly do a chest compression series (push way harder than you think you need to!) could save a victim's life if you roll up on them and they're not breathing.
Johnny Bresette shows how to properly execute CPR on a non-responsive, non-breathing victim. The key points? After calling 911, tilt their head back so their airway is as straight as possible. Interlace your hands, put the heel of your hands on the victim's chest while keeping your arms rigid, push down with your entire body weight and not just your arms, and do 30 compressions at the pace of the song Stayin' Alive (then appreciate the irony), then two full, deep breaths five seconds apart before resuming compressions. If you're alone, and untrained, stick to just compressions.
Keep doing this until better-trained assistance arrives or the victim comes to.