Here's how I do it. From flat level flight, pull up and do a half loop. At the top you will be inverted and when up there move both sticks towards the antenna (full throttle, right rudder, left aileron and down elevator). Depending on the plane, it should spiral about two revolutions then settle into a nice spin. My H9 Cessna 182 does it suprisingly flat although some planes I've seen didn't seem to settle into the spin as flatly for whatever reason.

- Joe