I feel like this thread is full of kids who can barely slide a rail/box and are trying to give advice.
1)The approach... Street-style rail ? Approach from the side where the leading foot is the one farther from the rail.. Normal rail ? Approach dead center as if the rail was between your legs.
2)The hop on... JUMP WITH YOUR TWO LEGS, at this point you can look at the beginning of the rail but this will change once you are actually sliding it.
3)Sliding stance... The back foot is ALWAYS flat while the front foot is at a slight angle. Get that cowboy stance to maintain proper balance between front and back leg.
Not as exaggerated as this but you get the idea. Now that you are on the rail and properly sliding it, you want to get to the end and clear it. ALWAYS look at the end of it. Just looking at it will let your brain know what to do to maintain the appropriate balance to reach the end you are desperately looking at.
This is for real beginners who don't have a single clue. Once you get more comfortable, lowering yourself on the rail will help you reach a lower center of gravity thus giving you more balance. Also, to any rotations in, on, off a rail,
is the key to success, even for the slightest rotation.