Native Girl on Horse

How do I get that Native girl to set on the horse in Chapter 3?

I'm not so sure what your asking, but i think you mean how do you get the girl to move with the horse? To do so, you would have to set the girl's vehicle to the horse. If you meant how to make her mounting up onto the horselook even somewhat decent, it's just a matter of patience, as that will take many small adjustments to get it right.