If I remember from flying the Pitts S-2B, entry into an inverted flat spin was fairly easy, starting from an entry in the vertical, similar to performing a hammerhead stall, but using a excess of rudder with insufficient aileron pressure. Another method is the fly a Lomcovak, which inevitably terminates in an inverted flat spin.
Again, it all depends on the edge of the envelop characteristics of the aircraft in question. And some aircraft may not be recoverable at all once a flat spin is entered. With aircraft certified to fly this kind of an aerobatic maneuver, recovery is fairly simple with motions being countered with rudder and elevator pressure.