1: go to a high place

2: walk to a ledge (at least 5 blocks down)

3: walk off the ledge

4: activate the bucket while holding the hand with the bucket at your side, as directly below your head as you can, you should now be floating in the air on the bucket

5: move around

IMPORTANT: when going down, toggle the bucket off and on, but don't let yourself fall too far each time or you'll take damage

Extra note: Once I flew into a wall and got my head stuck in a wall, deactivating the bucket while I was stuck allowed me to fly around without the bucket at a fixed height, I'm not sure if this is producible though, as I haven't tried to do it again, attempt at your own risk.