First clean and sand the plastic to prep the surface.  Bare plastic will require an adhesion promoter.  Which promoter depends on the type of plastic, and even more so, the type of paint you are using.

If the plastic has a painted finish, just some gentle scuffing will do.

Then when you paint it is just as painting anything else. Be sure to use a good primer.  If the plastic part can flex you will need to add a flex additive to the paint or it will chip and peel when the part does flex.

 

Again, the type of flex additive depends on the type of paint being used.  BE CAREFULL OF PAINT FLEX ADDITIVES (and some paints)!  Some of them are VERY toxic and you need a good resperator and/or and outside air source to a GOOD mask.  Others can be used with a average paint mask/resperator.  The ones with the filters attached.

 

So how do you know which is which?  Go to a good paint store that handles auto products.  Some NAPA stores do this, others do not.  In Akron Geo Folk is a good store, but there are many other very good ones.  Can't remember the name offhand (Masons?), but there is one in south Akron on RT 241 that is excellent.   You just have to find the ones in your area.