Practice flat turns and knife edge.

Practice slow rolls.

Slowly combine the 3. Do a 360 where you are level the first quadrant, knife edge the second, inverted the third, knife edge the fourth, and exit upright.

At least that's what I've heard, I learned to do them simply through practice and trial and error, which is just as valid It's a big deal getting used to seeing and controlling your airplanes in unusual attitudes, and you have to find your own comfort zone.