I enter a flat spin inverted, climbing steeply and at 1/2 throttle. Give it full rt rudder, then lots of down elevator (still neutral ailerons), then feed in right aileron until it flattens out. If I apply too much aileron too soon it flips out of the spin. Barry's method also works for me but I lose less altitude my way, not requiring the "wait for it, wait for it" part.
At our club flat spins are THE thing. One guy does spectacular invertad spins with a Cougar with oversized control surfaces. Nothing is better than my Burrito. It will flat spin upright also and climbs if I gas it..

For an upright flat spin, enter upright, climbing steeply , give it full left rudder, then a lot of up elevator, then feed in right aileron. It takes practice to get the timing right.
My UCD does rising inverted flat spins. I don't recall ever getting it to flat spin upright.
You can practice flat spins on a simulator. I use the Yak or Ultimate on APD and it works pretty much like real life, except most plane lose altiude faster on the sim.