A flat spin can be done on any kind of jumps and ramps. When your car hits the jump or ramp press your E-brake (x button) and push your left stick left or right. This will put your car into a flat spin. The amount of flat spin your car will get depends on your speed when you hit the ramp and how you hit the ramp. A lot of height and airtime might be good but it seems that the spin velocity is more important. The velocity at which the car spins mostly depends on how you hit the ramps and the timing of hitting your e-brake. With practice you should be able to pull off some amazing jumps.