For a RHG:

Place a watch or a couple of elastics on your left wrist. Slip a comb underneath the elastics so that the comb touches the back of your hand halfway between the wrist and knuckles. Now, don't bend the comb, and don't break contact with the comb during your backswing. Ignore your right wrist - it ends up cupped at the top.

Somewhere else, someone had said that when you cock your wrist up and down, you see the clubhead go up and down, but stay square. When you move your wrists side to side, you see it open and close the clubface.

So you need to have a flat left wrist that cocks up and down only. Let your right hand and wrist go along for the ride.

And forget about when to cock your wrists - if your backswing is sound, gravity will pull the club and cock your wrists for you.