Fairly simple - after speaking to Jan and asking to know more about Stealth, he'll take you around the body of water to a stack of boxes and ask you collect the piece of fruit he lays on the ground without being spotted. You can crouch (LB by default), so just creep around the boxes and collect the fruit when he's not looking. Achievement will pop shortly after grabbing the fruit. If you get spotted, just speak to Jan again to start the tutorial.