Depends on the car? The make the XRT/XRG oversteer, then you dont need to change anything. Just use [Hard Track], and when coming out of the corner, either accelerate hard, or just be rough with the car. Eventually this will unsettle the car, and will either cause understeer, or oversteer. It all depends on how your driving it.
The make the XFG oversteer, then just stick some knobblys on the back. That should do the trick
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To make the FBM oversteer (not sure why you would want it to), just turn down the rear downforce. But be warned, its hard to keep the car under control when it oversteers, especially with no downforce! ;d