Just get into a conversation with her about things that turn you on. Have her tell you hers, and then mention to her that you have a thing for feet. I would think it's kind of cute and I'd be curious about it, but I don't think it would really turn me on or off.
I would be fine with it, flattered, of course provided that he thought my feet were sexy and looking at them got him turned on. The girl would have to be pretty close minded to get upset about a foot fetish - that's a pretty normal fetish IMO.
DO NOT use the word "fetish" to her! Tell her you think HER feet are sexy and feminior whuteva you like about them. Or you can just do what the first anon user guy answered. She might like it and it will be something she won't mind, you will just do it without even talking about it. Win win. Sometimes the talking kills the fun.
Maybe she has a fetish as well. Try bringing it up in a conversation, but don't start a conversation about fetishes in an environment where it could be weird. Lead up to the fact that you have a fetish for feet. Maybe she'll say something like: 'Oh my goodness! no way, I have a fetish for (something)!' You never know. I don't have a boyfriend, because I'm a guy, so I can't really answer your question. All I can say is, good luck, and stay calm, cowboy!
Agree with anon. Of course it depends, if you have an old school 'fetish' and need it all the time, you'll need to bring it up at some point. But if you're just into feet on par with breasts or asses, then just ... include them in sex. etc.