rapid fire thoughts:
you want the centers of your feet an equal distance away from the center of the rail. (hard to explain i guess heh). but if you consciously think about one foot being 6 inches from the rail then the other should be also (that is probably about the right stance, a foot between your feet) but if one foot is 4 inches from where the rail is under you than the other foot should be also (8 inches total, 4 on each side), etc.
to get sliding any distance you obviously you need to have your balance - good practice would be riding 1 mph towards the rail get into the "slide" but stopped and just learn where you need to be over the deck to stay balanced
try think about it like a regular boardslide but the back of your shoulder is whats facing forward
practice slow and low then speed it up and get higher rails
good luck!