I agree with Jules. No need to come right out and label it your fetish. Tell her she's hot. Tell her she's gorgeous. Make it clear you fancy her. She'll get the idea - that you generally like the way she looks now.
If you want to feed her in a sexual way, you can introduce that idea without having to explain it goes along with a fetish. Lots of people think that food is sexy - try it first with something that "vanilla" people might do, like strawberries dipped in chocolate, or indulgent food you bought her as a gift, and see if she enjoys it. Or just get up earlier than her and make her waffles or pancakes smothered in butter and syrup and bring them back to bed, have breakfast in bed, and have sex straight after, caressing her full tummy. From there you can maybe start merging the two the next time, make it a domination thing, tell her she has to eat it ALL up or there'll be consequences.
I'm not sure an announcement is the right way to go - just act like you enjoy her figure (that should be easy), give her compliments, every time you have food together, spoon her a mouthful of yours telling her how delicious it is.
After all, you're saying this is a new thing for you too - why not wait and see how you enjoy things like this first. You never know, it could be a passing thing - or the way you feel about weight might be specific to her. You might just really like the way she looks right now rather than every chubby woman...?
(Sorry, I'm not trying to patronise you, I'm sure if you say you have a fetish then you do, but y'know, people do get super into stuff and then go off it sometimes. Every time I fall in love with (say) a thin man, a fat man, a man with a beard, an angular man, a toned man, a tanned man, a pale man... I think I LOVE that thing... but usually it turns out I just like THEM not beards/fatness/angularity etc in general. If that makes sense. And the next person I fall in love with is completely different. Sorry if you're 100% sure fat/weight gain/feederism is your thing - it just seemed from the way you were telling it that you were only recently discovering all this stuff - which is cool and amazing if you are).
Good luck, have fun