what do you want? some kind of guide...."how to make your girlfriend into interracial sex"?
I don't think there's anything like that...there are no rules in sex.
If she doesn't want, you can't do absolutely nothing
Start watching interracial porn with her. If she seems to like it, start watching interracial cuckold porn or IR porn where there is a definite theme of white penis inferiority, like where girls talk about how much better black dick is. Make sure you're eating her pussy while you watch. Start dirty talking about it. Get her to admit that the big black dick is beautiful, or that it turns her on.