Stall model keeping nose up with full thottle and full up.

Initiate spin with rudder then feed in some opposite aileron. If you have enough movement, power and the CG is far enough back then it should flat spin.

It is also possible to get a flat spin if the fin area is too small [ I am told ]. Never had it happen to anything I flew but there are full size aircraft which can be made to flat spin and it may be difficult to get back to normal flight.