how do you "picture" outfits when shopping?

whenever i'm out shopping with my girlfriend, it seems like she can immediately envision a perfect outfit for any piece, and can make any piece look good in theory. she can really build up an outfit with things mentally catalogued in her wardrobe and including pieces she would need to buy or supplement, and make substitutions wherever she sees fit. i've tried asking her how she can do it, but all she says is "it takes practice" !! but, i don't even know what to "practice" in the first place!

whenever i see an individual piece, it feels like my brain stops there and i can't see the rest of the picture. if i don't like how the piece looks on its own, i have no interest in it and can't picture it in a full outfit, and she will point out that it's the exact thing i love in fashion and would go perfectly with outfits i've liked on pinterest. more often than not, she's completely right.

i really want to get better at shopping and dressing, but i have no idea what the most efficient way is to start learning how to do so when i have such difficulty picturing outfits in the first place! so, how do you go about "picturing" an outfit, and what helped you learn?