There's no need to tell her anything, just show her. Next time you're on the couch together, put her feet on your lap and start rubbing her soles with your thumbs. Give her an honest non-sensual foot rub like you'd massage her shoulders and back to relieve tension. She'll associate your touch on her feet with pleasure. Do that a few times, then at the end of the massage try kissing her toes playfully and see how she reacts. She might laugh but that's okay. You want her to feel good and be comfortable. If she doesn't tell you to stop, try again a little more passionately and go from there.
The next time you're having sex, after you're both worked up and have been doing it for a bit, try sucking on her middle or index finger. Later, try sucking on her big toe. Then, you can work your way down to the little toe, one at a time. Don't go crazy and shove her whole foot in your mouth or slobber on her soles, she should be fully aroused first. You may be able to work up to that later if desired.
There is no need to feel ashamed. There is no shame in appreciating her body, feet and all. Most women in my experience like it even though they're not necessarily "into" feet. She'll most likely be into you worshiping her body, head to toe. If you're willing to go down on her and make her climax, she'll probably be willing to return the favor with her feet.