Joe B gave one good way. Here is another VERY EASY WAY to do it. Take the plane straight vertical, go up a ways. Back off the throttle to idle. Just before she starts sliding backwards, take the rudder/throttle stick to the lower right corner and put the ele/aileron stick in the upper left corner.

At this point you are full down elevator, full left aileron, full right rudder and NO throttle . This will start the inverted spin. Let it go for a couple of revolutions then make it flat.

To make it a FLAT spin, start adding throttle AND creeping the ailerons over to the other side . How much throttle and opposite aileron is up to you and the plane. Leave the rudder all the way right and the elevator shoved forward. If it is capable of a flat spin, it will flatten out nicely with opposite aileron and the addition of some throttle.

I have a planes that will flat spin like crazy doing exactly this. Some will enter in many other ways, but this way will work for most anything capable.

-Cheech