It's not especially complicated. The main gameplay works just like CS1, fighting when you attract attention, so not really "stealth," though you can sometimes avoid a fight here and there. If you get caught repeatedly it switches to a basic stealth mechanic. Normally the guards move randomly, but once you're in this new mode they move in basic looping patrols, and have a somewhat smaller zone of where they'll see you. So it's just a matter of correctly timing your movements to get through these patrols. If you get caught you reset to the start of the area, and if you get caught too many times it's a game over.
So not too hard, and it's entirely possible to play the game and not encounter the stealth mechanic at all, depending on how you play.