Start with little things like increasing her portion sizes subtly, swap ingredients in food for higher calorie options like higher fat variants, when you're with her get her food like snacks and fast food and when you're having sex or doing anything sexual, grab her belly and other fat parts. The last two should start to make an association in her mind of having food when you too are together and fat and sex being intertwined.
Classical conditioning can be powerful to really get someone into this fetish. That's why some people get wet or a boner when they eat. If you want to take it a step further, feeding her during sex and/or while she's stuffed will make her associate it more with sex. One thing I do with my gf is when I'm giving her backshots or fingering her I play with her belly and other fat parts.
And it goes without saying , make sure everything is consensual because if you go ahead with it, even if you break up she'll still have this fetish if all the conditioning stuff works