PRESS THE SAME BUTTON TO FEED AS YOU DO TO MOUNT, and feed him as soon as you mount! For me on Xbox, it's Y. Then press again all times he tries to buck you off. Lost a unicorn because I didn't know what button would feed him, I thought the mounting button would make me dismount.
You can feed it then use a bola (which doesn’t lower effectiveness) and after that use fence foundations and walls to keep it in place and ride it for the duration of the tame. No need for it to run around randomly.
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.