sand them down, them use primer, use tape if u want a design, let the primer sit for a day, then paint it, let it sit for a day, then put a clear coat.
^like mentioned, make sure you sand with some fine sandpaper, prime, let dry, and paint. Do it in a 20*c(ish) temp, dry area free of dust. Also make sure the primer and paint is meant for a plastic surface and that it has enough flex agent in it so it won't crack... and use a few coats of clear to make it shine and protect the paint. Also with lots of flex agent.