I humbly disagree
I think that if you were going to feel comfortable sitting her down and telling her, you'd have done that already.
Here's what *I* suggest. Don't tell her, show her. Remember, the girls like take-charge guys. Next time you're sitting on the sofa watching a movie, casually tell her to put her feet in your lap. If she resists, just coax her along, maybe reach down and pull them up. If she STILL resists, stop (and draw your own conclusions about that). If she doesn't resist, just pull her shoes and socks off and start massaging her feet. Judge her reaction -- is she tentative, enthusiastic, or somewhere in between? Don't tickle right away. Just rub -- and don't stop after 5 minutes. Rub for the rest of the movie, or until she tells you to stop. Don't make it your main focus, but don't do it too casusally either. Multitask between watching the movie and rubbing her feet. Give her the best foot rub you can. Don't do it half-assed because you're tentative about the situation. Committ, may man, committ.
Just be sure to look for her reactions and respond quickly. If she purrs, smile and say "Like that, huh?" If she gives an enthusiastic response, tickle lightly, smile and ask "Like that too?" Let her responses guide you.
The bottom line is that if you play with her feet enough (like every appropriate opportunity that comes your way 🙂, she'll figure it out on her own. She may even ask you straight out if you have a foot fetish. Whatever you do, when the question comes, DO NOT react in a sheepish or embarrased way. Instead, say something like "uhhh, YEAH.. couldn't you tell?" Smile and make sure she sees that YOU are comfortable with it. If YOU think it's creepy, there's no reason why she shouldn't too. On the other hand, if you are fine with it and take it in stride, she'll get the message that it could lead to some great sex. Believe me, that will be all she needs to accept your foot/tickle fetish. UNLESS:
1) She's a total control freak, in which case you can forget about the tickling. Loss of control is not too popular with control freaks.
2) She has some deep adversity to feet. If that's the case, you're out of luck entirely. My experience is that you can work your ass off in reassuring her, telling her that her feet are cute and they turn you on, blah blah blah... But chances are she's not going to come around.
My two cents' worth.
-Pete