It was a really long time ago, about 7 years now. She had never had a single sexual experience before we were together, so she didn't know ANYTHING. I hate to use the word "teaching", but it was pretty much what I was doing.
I was 18 back then, so I was still SUPER shy about my foot fetish. She heard the word fetish from some MTV show we were watching and, while she knew the literal definition of the word, she didn't really know what it was. I explained it to her, and then told her I had one. I let her guess what it was, mostly out of shyness. She eventually landed on feet, to which I confirmed it. She thought I was joking, but then said "Hey I love foot massages, you can do whatever you want with them". From then on it was a pretty slow climb to me letting on just HOW much I liked feet. We didn't have sex for awhile (Not wanting to get her pregnant), so we stuck with handjobs for awhile. She would let me suck her toes sometimes, but would always keep her feet really clean.
Over time I just let more and more on, How much I want to be tied up and dominated by them, how I also want to just smell them, how I want them to be as sweaty and smelly as they can be. A big turning point happened a few years ago. It was Valentines Day, I had taken her out shopping in her flats. Her feet don't usually get very sweaty, but I took her shoes off in my lap and could feel how moist they were. I couldn't help myself, and just buried my nose in her soles. I had a huge erection and was basically in a lust trance while sniffing her feet. She said something like "Oh wow, you really like smelling my feet huh? You can do it as long as you want". I think that was when she fully understood how much I loved her feet. She has taken it all pretty well, and at this point she knows about all there is to know. She just brings it up very casually now, and is completely ok with me licking or smelly her feet even in non-sexual circumstances.