John,
Enter a flat spin by starting with a conventional spin. Then slowly reverse the aileron input. Some planes will enter the flat spin with neutral ailerons where others need opposite aileron. The most common mistake is not having the wing fully stalled when entering which results in a downward spiral. The plane must be set up with sufficient throws and a CG far enough aft to flat spin well. If you can locate Dave Patrick's "Wring it Out" series tape #2
he goes into this in detail both upright and inverted with split screen images. Bill

P.S. A really cool spin which many don't fly is the "Knife Edge" spin. It comes down on the side of the fuselage with the wing vertical. Enter from a conventional upright spin and quickly reverse the elevator to full down. CAP's knife edge spin better than any other plane due to the forward placement of the stab and the tapered leading edge.