Positioning your legs. For a correct stirrup length - there should be 1 inch from the bottom of the leaping horn to the top of your thigh. You should sit straight up and further back than if you were riding in a normal hunt or dressage saddle. If you are accustom to riding saddleseat (which is what I ride) you already ride sitting straight and further back
As for mounting; I have been told that when you begin, it is easier to mount as you would an astride saddle and then swing your right leg back over the pommel while keeping your back and behind straight.