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.
This stinking thing!!! So annoying!!! I was taming it on a cliff and I was a bit slow and it knocked me off. RIGHT INTO A SWAMP. I swear, while everything was eating me, sarcophagus, titanoboa and mega parana, I saw it laughing at me.
They are mostly found ( in my experience ) Where Carnivores are ( Raptors, Carnos, terror birds etc. ) So you really have to make sure the area is safe enough so you can hop on the equus and feed it. I hope this helps!