I spent three winters looking like a marshmallow. Not the cute kind. The kind where your arms can't bend and the back of your coat rides up over your hoodie every time you sit down.
I thought the problem was the coat. Too small, I told myself. So I bought a bigger coat. Then I looked like I was wearing my dad's jacket over my little brother's hoodie. Worse.
Finally, a brand rep I work with—guy named Marcus who dresses better than anyone I know—watched me struggle into my coat outside a coffee shop and just said, "Dude. Stop. You're doing it backwards."
Here's what he meant.

The hoodie has to be thin.
Not your heavyweight winter hoodie. That one stays home or goes on top. For under a coat, you want the light one. The one you barely feel when you pick it up. My go-to is an old Uniqlo fleece pullover that's probably five years old. The fabric is worn soft. No puff to it.
If your hoodie has a thick front pocket or a big chunky zipper, it's going to push your coat forward and leave a gap at your neck. Cold air gets in. You look bulky. Bad both ways.
The coat has to be one size bigger than you think.
Not two sizes. One. You want room in the shoulders and chest so the hoodie fabric has somewhere to go. Try this at a store if you can: put on your hoodie, then put on the coat. Raise your arms like you're grabbing something off a high shelf. If the coat pulls tight across your back, it's too small. If it bunches up around your armpits, also too small.
I made this mistake with a Carhartt WIP Detroit Jacket last year. Bought my normal size. With a hoodie underneath, I couldn't close the zipper without sucking in my stomach. Embarrassing. I sold it to a coworker for half price.
The hoods have to stack, not fight.
This is the thing nobody talks about. Your hoodie hood and your coat hood are going to sit on top of each other. You have two choices: tuck the hoodie hood inside the coat (which looks clean but feels weird against your neck) or let it sit outside.
If you let it sit outside, make sure your coat hood is bigger. A small coat hood over a big hoodie hood makes a lump behind your head. You look like a turtle. I learned this walking home from the shop one night. Sophie saw me from across the street and laughed. She thought I was wearing a neck brace.
Colors matter more than you'd think.
Dark hoodie under a light coat? Fine. Light hoodie under a dark coat? Also fine. But don't do the same color unless it's black. A gray hoodie under a gray coat just looks like one blob. No shape. No contrast. I did navy on navy once and a guy at the flea market asked if I was wearing a jumpsuit.
What I actually wear:
Hoodie: Uniqlo U lightweight sweat (olive or cream, both work)
Coat: Carhartt WIP Clash Parka, one size up (black)
Or swap the coat for a vintage denim jacket if it's not freezing
The parka is roomy without being dumb. The hoodie is thin but warm enough for LA mornings. Together they move when I move. I can skate. I can reach for something on a high shelf. I don't look like I'm smuggling pillows.
One thing I still mess up: I forget to zip the coat all the way. Then the hoodie peeks out at the bottom and it looks sloppy. Sophie points this out every time. I tell her I'll remember next time. I never do.
Anyway, go try your hoodie under your coat right now. Raise your arms. See what happens. If you feel stuck, switch to a thinner hoodie or a bigger coat. Or just wear the hoodie alone and throw the coat over your shoulder. That works too.
Less thinking. More moving.