After a series of cut scenes you will find yourself facing a couple of horses, walk up to yours (the one on the right), and press Y to mount it. Follow Tom to the center of the meadow and he will give you a basic tutorial about how to ride your horse. Once he has finished teaching you the basics he will lead you to the start of the race, stand inside the gold circle facing the direction as Tom, as soon as he says Go you will need to race him around the edge of this area for three laps, it is a fairly straight forward race with a couple of jumps, but as long as you maintain some speed by spurring your horse with LB and stick to the main route you will be able to beat Tom with ease, just be careful not to spur your horse too much or it will die and you will need to restart from the beginning of the race. Once you win the race Tom will ask you to follow him to the center of the meadow again, just follow his instructions, and once you finish with the bottles some wolves will come and attack, kill them and a cut scene will occur, once the cut scene ends you will need to kill Tom and his two allies, and when they are dead the achievement will unlock.