The way i make them is by not using just putty. Putty should just be used to fill gaps and for curves and contours. To get real definition and shape u will need to build a basic 'skeleton' with sheet styrene and any other styrene shaped strips u find useful. Study the photos of the real bodykit to work this out. Keep as much of the original bumpers etc on your model but cut away unecessary areas ie large front intakes etc. This is just my method, i find it works well for me but others have their own way.
My very crude drawing delow will hopefully give u some clue as to what im on about.
