Honestly, the best thing to do is just ask. People are generally far more willing and open to try things out with those they’re being intimate with than your realise.
I’ve broached the topic with several partners to great success, it has generally followed the trend of me offering foot rubs, sucking toes during sex and eventually the topic of feet being incorporated into intimacy is brought up. Give it time, lead into it. As long as there’s trust, communication and a healthy deal of self awareness then there shouldn’t be an issue. If the person isn’t into it, then they aren’t into it but at least you have the peace of mind knowing you’d asked and not dwell on the mystery of ‘what if’. I’ve discovered that the women I’ve been with are happy to oblige my fetish since I do the same for theirs and it turns out, some of them really enjoyed it too!
Don’t get too caught up in the logistics of the approach. Intimacy should be fun for all parties involved and if you’re with someone you can vibe with then I’m sure they’ll grant your desire. The worst thing to do is get lost in the analysis paralysis. Be present with them and, assuming they’re as present as you, things should happen naturally.
Just remember to have fun, nerves can be smelled from a mile away.