Once again you need to head back to Bonnie's house to start this mission, this time however, her father will be joining you for a spot of 'horse breaking'. This involves riding the horse 'buck-o-bronto' style to tame it.
But before you get the chance to do that, you need to head outside and follow the two of them on your horse until you reach the field full of wild horses. Your lasso will already be equipped, so when you get relatively close to a horse, hold down LT/L2 and then press fire to get the lasso around its neck.
Now hold fire down until Bonnie or her dad also lasso's the horse and then jump off your horse and climb onto the new one. It'll start to kick and flail around, so you need to press your stick in the opposite direction of where John is falling (to balance him out and keep him on the horse).
This will go on for around 20 - 30 seconds, until you manage to stay on and tame the horse. If you notice that John's arms are starting to flap around, this is your sign that he's close to being thrown off, so quickly press in the opposite direction to keep him on!
Once you've tamed the first horse, you now need to repeat this step for a second horse, only this time when you lasso the horse, keep holding fire down to keep the rope - and the horse - steady before getting off your horse and climbing up on this one.
Again you need to tame it by pressing your stick in the opposite direction that your horse tries to throw you. Now dismount the horse once it's tamed and then get back on your horse before following Bonnie home.