(If you want to do this mission immediately after the last Bonnie mission, then simply run outside Armadillo, set a waypoint for Bonnie's house and use a campsite to warp to it). Run into the familiar yellow marker and then run to your horse once the cut-scene finishes.
Follow Bonnie to the cattle and start from the yellow spot in the field, before riding your horse slowly behind the cattle. Stay behind them until they leave the pen, cross the railway line and go up the hill.
The trick to getting the herd to go where you want them to is to aim yourself in the opposite direction of where you want them to go. So moving to the right of them will make them turn left, moving to their left will make them turn right. The mission will end as soon as you get them to the big oak tree.