How DO I Tell My Girlfriend I Have A Foot Fetish?
Some folks are funny about feet (a shame I know), but even if she did not share the fetish herself, it would be very rare that someone was so fazed as to call off a relationship.
As for telling her - have you thought about actions rather than words? Perhaps giving her a massage, starting at her shoulders and neck, working down her back, then doing the thighs and carrying on all the way to her feet.
If she starts to freak out as you get near her ankles then you might need to rethink what you are going to say/do next. If she seems to enjoying a sensual massage, she might be quite happy that you seems to spend a LONG time with her feet.
If she is happy with this, then it is for you to decide if you should do more than just massage her feet - as a general rule I have found if someone lets you massage something for a long while they usually have no problem in letting you kiss/lick/stroke/such there as well.
If you feel unsure about this way, then a simple flattering comment like "I do like the way you do/do not use nail varnish on your toes, you have such pretty feet, blah blah"
What feels the best way for YOU is what matters, I am just offering a couple of ideas for you to consider.