I enter inverted flat spins with the up and over technique as I call it. While going vertical, chop the throttle and when it's almost slowed to a stop I add in full down and full right rudder, since mine flat spins inverted better to the right. This gets it into the spin almost immediately with my Goldberg Matrix. I then add a touch of power. My plane doesn't like ailerons in flat spins. So planes do.

Mike