How do I sand it paint on my front bumper? I don't want to damage the plastic bumper, but I want to take some of the areas of flaking paint off. What grit sand paper? Etc...

It depends on how you are going to sand it. By hand or sander? If you stay at 120 grit or finer you shouldn't do any damage to the plastic that can't easily be primed and block sanded out. I say that presuming you have a moderate amount of common sense. Some people can screw anything up, given the chance. If you are sanding with the idea of refinishing the existing paint without a lot of priming then you should finish with something in the 400 to 800 range depending on the paint system you're using and quality of the job you're looking for.