You won't
guaranteed crack a rib, but it's a good bet (watch that first video I posted; they're pretty rough on the gal). As long as you don't puncture the heart, they can fix anything else later. Assume you're starting with a corpse (no breathing, no heartbeat, brain death in 4 to 6 minutes if you do NOTHING), the worst you can do is damage a corpse a little. The best you can do keep 'em alive until the EMTs arrive.
Current protocols are you check for a maximum of 10 seconds to see whether they're breathing and have a pulse. If you've never practiced checking for a pulse, now's a good time to try it. Tell your friends you just want to cop a quick feel...
The carotid artery in the neck is easiest for most of us, followed by the radial in the wrist:
The first aid books I have also mention using the brachial artery in the elbow, but I've never had good luck with that one, especially on heavier people. It's OK with a stethoscope, but you won't have one of those handy.
JBRG, the only changes to the protocols since you were certified are: compression depth of 2" to 2.4" (5 to 6cm), and 2 breaths per 30 compressions at 100 to 120BPM. 'Stayin' Alive' is still an acceptable rhythm, although Queen's 'Another one bites the dust' is dead-center at 110BPM, compared to 104 for 'Stayin' Alive'...
https://www.youtu.be/watch?v=I_izvAbhExY Stayin' Alive
or
https://www.youtu.be/watch?v=rY0WxgSXdEE Another One Bites the Dust
(try to avoid humming this one as you're working, as it sends the wrong signals...)
Additionally, heavy emphasis on spending as little time as possible doing the 'breath' part, as it takes 7 or 8 compression cycles to get the blood flowing again every time you take a break to breathe, so you're wasting nearly 1/4th of the compression cycles. They recommend having someone else do the breathing part while you do 200 to 300 compression cycles, then the two of you switch places.
Finally, emphasis to make sure you fully come off of them during compression 'up' strokes, allowing the chest to rebound. Leaning on them cuts the efficiency by up to 30%. If you watch the guy in the car accident video, it looks like he's not coming all the way up. Doing
anything is better than nothing, but as the efficiency goes up, so does the survival rate.
One thing most people screw up in CPR training is the 'breath' part. Notice how you're breathing right now? Yeah, that's a breath. You don't want (or need) to 'inflate the chest'; you could cause other problems trying a big breath, although it's hard to control when you're pumping adrenaline working on keeping someone alive.