I am a custom automotive painter by trade so here is what I suggest:
Mask off any areas you don't want painted including under the drive door. You don't want over/under spray getting everywhere.
Use an abrasive sand paper at 600 grit and scuff it until the plastic is dull. Take care to get into creases and around edges and such.
Use a plastic adhesion promoter. You should be able to get this in a spray can.
Use a paint suitable for automotive applications. Can be a spray can if you don't have access to a spray gun etc.
Once it is the colour you want, use an automotive clear-coat to protect the paint. You should also be able to get this in spray can form.
That is basically it.