Slowly introduce watching porn together before sex, as a start. Then a video that might have one or two interracial scenes. See if that gets her excited. Then a porn vid featuring white guys with big cocks and size queens. Then a full on interracial bbc vid. Then another mixed scene vid. Then maybe an interracial vid with the white hubby or bf watching. Take it gradual unless she starts to take the lead. Get some ideas into her head and see how she reacts.
Those guides above are great suggestions and have a bunch more detailed advice. Though I know quite a few white girls that got into the fantasy themselves simply by stumbling across interracial porn.
Re: The guide with steps. I think the advice of not having vaginal intercourse with her isn't good advice. She may become bored with you and leave the relationship or not think you desire her. Don't push her away.
@Jeffgamer1989 I am not sure I fully understand what you are seeking. It seems you need more than just advice. You may have some masturbation material for yourself, but if you want to keep your gf - your gf and not lose her, you will need to grow a pair of balls....im sorry for being so harsh on you jeff, but seriously, the last 5 guys that i have talked to on skype, have all broken up 3-4 months after they "think" they are moving forward with this so -called fantasy, because you took it too far on your own and you didnt tell your gf what the real deal was.