I drive a black BMW and this question is not easy to answer. I use a rotary and orbital to get most all the swrils out. If you do not have any of these or no how to use them, then hand applications will produce some decent results. The products that you use are also a key factor.
Question how old is the car, how do you wash the car currently, how much time do you want to spend, and do you have any tools at all.