This is how I learnt it. For right side spinners:
Get backflips down and learn to set the flip with your hips.
Then instead of thrusting your hips straight forward, start gradually setting them a bit left of center. This will send you into off axis backflips or flat 3s.
Once you have those down just exaggerate it even more to the left and bring that last 180 around.
It's best to learn that progression on a tramp beforehand. This is also where the difference between a flat/rodeo 5 is negligible.