Well as someone else said you want to try & roll onto your shoulder NOT head! Put your arms ahead of you and tuck your right shoulder (if falling off the right) or left shoulder (if falling from the left) under so that your shoulder takes most of the impact. You don't want to try & land on your feet (as I tried once lmao) because the momentum with send you flat on your face & probably leave you with a sprained or busted leg.
This roll isnt always easy to perform because you're not always
prepared to fall but i've so far been able to do it for most of my falls through the years & on 2 occasions actually ended up back on my feet lol!
Of course after the fall always GET BACK ON. If not, then your horse can get bad habits & you can get timid the longer you have to think about it. I've had the wind knocked out of me & my ankle badly sprained but still got back on even for just five minutes. ;)