The real question is what do the feet look like. Because not everyone has feet that should be near your mouth. And even if they do have feet that don't look like they've been walking on glass you should still proceed with caution.
If you're still gonna dive in here are some things you should do:
First off you will slowly want to remove their socks and shoes. You can do this very nicely. If they have on socks, remove them with your teeth by grabbing the end and pulling them off. This will help to set the move for what is going to come next while you learn how to lick feet.
Slowly start to lick the tip of their toes. You will lick the tips and then move slowly up each toe. Lick back down the toe moving your tongue gently. Don't forget to look up in their eyes from time to time. You can also gently kiss their feet or toes for an added pleasure. Always use your tongue and your mouth. Do not use your teeth. Even a gentle bite does not feel good on feet.
Sucking on toes is very erotic. Start by taking one entire toe into your mouth and sucking on it gently. Start to suck harder and if they really enjoy it you can place more than one toe in your mouth.