Easy way to get a flat spin (Inverted)....

Push the sticks to the antenna. If the plane is in a vertical dive shove both sticks to the antenna. Up and in. The plane should immediately enter an inverted flat spin.

Modify this to get a blender....

Start with a vertical dive. Give full left aileron only. Let plane complete a few rotations. Then, shove the sticks to the antenna. Hold on! Blender to flat spin.

Phil