Hi

1st you need a well set up model, for example one that doesn't dive like a kamikaze pilot while inverted, it needs to be "neutral" in both prone and inverted flight.
Learn sloooooowwwwwww rolls and slow transitons from prone to KE to Invert to KE back to prone flight
once you can take 10 seconds+ to do a slooooooww roll, then progress onto trying it at height in a harrier, ie very little forward ground speed, remeber to use correct inputs of rudder, elevator and power.
Are yo sure no one in your club can help?

best regards

Hissing Sid