The way I do it is to climb up about 300', cut the throttle, nose over & dive toward the ground, give right rudder, left aileron & down elevator all @ the same time, after the plane makes about three spins throw your aileron to the right & it will go into an inverted flat spin, once in the flat spin on some planes you can ease the ailerons back to neutral & throttle up a little to increase the speed of the spin. Not all planes will do a good inverted flat spin. I have a sig profile ultimate that does great flat spins & I also have a 1/3rd scale pitts that will only make about two spins & it just stops spinning & falls, still inverted. You can use the same move with up elevator for an upright flat spin.
Good luck
Gil