Before we get started on the Back Deck Roll lets discuss the prerequisite skills and things you will need.
-Roll on one side (either sweep or C-2-C)
-A Friend with a kayak (will become clear why soon)
(assuming you roll right handed setting up on the left side of your boat) Hold your friends boat on your left side. Rest your head on your hands exactly the same as a beginner level hip flick drill. Push your feet away from you so your boats go from a T shape to something more like one long, wonky line. Now keeping pressure on your feet move your boat over your head, primarily using your hips and legs. During this drill it is important to keep your head low and not push on your hands/arms really at all.
Go back to the left side and do this drill again. Repeat about 100 times or until it is a smooth effortless movement, whichever comes first.
Once it is smooth with another boat we are going to introduce adding your paddle in place of the boat. Roll your wrists so the power face of your paddle is almost pointed upward. Go upside down your left side. Keeping your body pinned back, reach your paddle to the surface of the water in basically the exact position that your friends kayak was in. Now push down on the surface of the water and move your boat over your head.
If you actually practiced as much as I told you to in step 2 it should be a quick progression. If you were pushing through your arms or forcing your head up you may need some more practice.
And there you have it. How to back deck roll