Slip your sling over your weak arm (left for most shooters) and over your shoulder, keeping the sling behind your shoulder and the gun up front. If your sling is anywhere close to the proper length and your rifle is of conventional styling (i.e. not a bullpup or other radical departure from tradition), this will allow your weak hand to comfortably rest in the pistol-grip area of the stock.
Simply grasping the wrist of the stock with your left hand with finger and thumb will provide you with good control of the gun. So, there ya go. Your gun is up front and under control, and can easily and almost effortlessly be kept from accidentally slipping off your shoulder by grasping lightly with your off-side hand.
How easy is it to shoulder the gun from this position? See the next couple pages to find out.