Easy! Okay, so, once you've found Bonny and you're up to this quest, follow her and her father to the heard. Get your lasso out and aim at one of the horses, it can be any, I always go for the prettiest one c:
Anyway, once you have swung your lasso (LT) keep hold of it otherwise John will withdraw it from the horse. Keep very close to the horse, even when it runs, so that either Bonny or her dad can lasso it. Only let go when they've lassoed it.
Then dismount your horse and mount the wild one. Use the thumbstick to balance yourself. If you see him tilting to the left, put your thumbstick to the right. And vice versa. It's quite an easy quest, but if failed too many times, can and will get frustrating. Don't skip it though, there's a brilliant mount prize at the end of it!
Hope this helped, partner.