You first need to be able to do slow rolls with the proper rudder inputs. Then start flying around in a circle. At a comfortable point in the circle (probably when the plane is flying away from you), do one quick roll then keep flying in the circle. Keep flying the circle with one roll and try to force the plane into the circle and it rolls. Visualize the circle and force the nose to follow it using the rudder and elevator. I think it's easier to make a big circle and roll slowly.

Tom