It's hard because it's the top most license in the game! On the plus side, if you can win the licenses you'll be more than fast enough to be competitive.
The key is to watch the demos from the in-car view, and make a note of where the computer car brakes, what gears it corners in, and what lines it takes.
Don't even bother attemping the S-License until you absolutely need to. Just complete as much of the game/races that don't need S-License first. This will help you a great deal because you'll be used to the tracks and cars so when you come to doing it you should find it relatively easy by then!