Feed it then mount. You won't be able to control it until the tame is done, dismounting appears to lower taming effectiveness so make sure you have a good supply of carrots (preferred food) on you. When it starts to buck you off you have to hit enter to feed it again. If you pay attention to the bottom left corner of your screen there will be a tool tip that will pop up when you can feed it abs just before it starts bucking.
These guys are easy to tame low level but a 150! The tame percentage drops as the horse runs do good luck ever taming a high level one. Unless! You knock it out build a box around it wait for it to wake up and the tame to your hearts content. Vote up to help others!