Setup to the golf ball as you normally would.
Press your bottom hand into your top hand's thumb [Note: For righties, press your right hand into your left thumb] until the club shaft is parallel to the ground and the butt of the club is pointing parallel to the target line. Only move your hands while getting to this position. If this "pressing thing" doesn't work for you .... just move your hands such that the butt of the club is pointing down the line and the shaft is parallel to the ground. In this position, for righties, the back of your left hand faces away from you, whereas it should have been pointing down the line at address. Did you get into the right position ?
Good !
Well then just take a backswing from there with a nice big shoulder turn (back facing the target when you are done). Well ... see how high your hands are above your head ? That is where they should be at the top of swing position !

Hit lots of balls from this new top of swing position.
Of course, eventually, you'll swing the club back without this wrist turn from the beginning .... but this will get you feeling what a non-flat swing is like !
Once you've gotten good at that ... the bonus drill is to bring a wrist cock into your swing. At the beginning, try cocking your wrist earlier in the swing.

I hope this helps.
Good luck !