"It still shocks me! It's not even normal," she tells PEOPLE. "I still call him my boyfriend, and he, every once in a while, is like, 'Babe, I'm your fiancé.' "
While Duff, who is taking her Real Girl's Kitchen lifestyle brand to the Cooking Channel next week, continues to find the right words about the guy who popped the question on April Fool's Day, she does concede that the apparel entrepreneur – he owns Subway Tile Shirts – is the "one."
"He just complements me so well," says Duff, 29. "We're very different in a lot of ways. I think really when he became close with my family, and I saw how simpatico everybody was, that's when I was like, 'Okay, so not only do I love him, but they love him too.' And I feel like that's the way it's supposed to be."
Because she has been so tied up with her impending TV show – titled, appropriately, The Real Girl's Kitchen, Duff has been at a lack to play bride-to-be.
"I really haven't had any time to sort of Google 'wedding dresses' and Pinterest boards and all the stuff that people I think normally do," she told reporters while attending the 11th Annual Inspiration Awards benefiting Step Up. "But in about a week and a half I'm going to get into it."
Her plans for the ceremony call for the more relaxed, the better.
"We're pretty laidback, [so] I feel like it'll be easy and casual," Duff says with a laugh. "Not casual like, you know, linen pants and barefoot, but not super black tie. I want it to be a day [in which] everyone is very comfortable … which I think is what everyone says.
She added, "I don't think that ever happens, but that's what I'm aiming for."
Throughout, she says, her younger sister, Hilary Duff, is at her side. And Hilary's split from husband Mike Comrie last January hasn't put a damper on all that's happening.
"You know, I think maybe we all think it's a lot weirder than it really is," Haylie said of her sister's separation, "but we love Mike, and he's still so much a part of our family. I might see Mike actually more than I see Hilary, which is weird."
Asked for her thoughts on the possibility of they're having a reconciliation, Haylie said, "I think they're doing great, and they're just taking their time. They're young, you know? So they're not putting stress to figure out which way it's going to go."