My tip for writing believable fluff is: don't think of it as something weird. They don't have to be floating from one joyous event to the next and having little Eskimo kisses and petting bunnies all the time (unless your assignment specified that it be tooth-rottingly cute). Fluff as I understand it just means light and happy. It can be a PWP too (unless your assignment specified no porn!) It doesn't have to be something too long, does it? Just write about a happy, throwaway moment. A lazy morning in bed where one of them wants to get up and do something but the other just wants to snuggle and make out and keeps dragging them back. Maybe they talk about the angsty stuff currently relevant to your fandom and one of them gets angry about it and the other gets distressed by the intrusion of angst on their one day off and shuts them up with making out. Or write about them cooking together. Or have them babysitting and being terrible at it but thinking the other is cute when they're spoonfeeding the little monster and getting spit up on.

More tips: Don't feel you have to use pet names or the phrase "I love you" if that makes you uncomfortable. Don't try and write comedy if that makes you uncomfortable. Just find a happy moment you're interested in and write about it. I'll be honest, fluff isn't my strong point either so I steer clear of anything that isn't...realistic fluff, but I hope something there helped.