As a certified Popsicle, I agree with MM.
Heat management in the cold is difficult, plus it takes a while to find out what you really need.
A parka is easy to focus on but don’t forget the other things, like warm hats, wearing the right boots, and gloves, balaclavas and neck gaiters (invaluable and multi use), warm trousers/bibs, and wind goggles.
If it is cold or cold and wet, avoid cotton, synthetics work better and don’t hold the moisture. Keeping warm is simply about minimizing heat loss and making sure you are generating enough heat to stay warm.
So some basic tips and advise…
Wind shells keep the warm air trapped close to your body, making insulators. Loft like fleece and down trap air spaces. If it isn’t windy, open up your wind shell to help vent.
Food, don’t eat big meals, they take to much blood to digest and will make your feet and hands cool. Eat all day long, but little snacks. Try to balance protein fat and carbs, as you body burns them all different and the balance helps keep you warm.
More important than food, drink lots of water. Drinking keeps your blood volume up, helps you digest food, and keeps heat moving around in your body. In cold dry air you can loose half a gallon a day breathing. It is not unusual for me to drink 3-5 gallons of water a day when doing moderately physical activities in extreme dry/cold conditions. Drink continuously as your body can only take water in so fast.
Don’t tolerate anything getting cold. Deal with it right away. If you wait you may find yourself on a slippery slope, and all of the sudden you can’t help yourself anymore.
If your feet are cold, add more pants.
I generally like having a good spacious wind shell top and bottom, and layering up under that. I like heavier nylon If I stop and am sitting around, i tend to break out those compact able puffy down filled jackets to keep my core warmer, and big ‘camp gloves’ and hats for sitting around. Lightweight polypro is a good first layer, then fleece and/or heavy weight poly pro, then a windshell. I try to regulate heat out of the top of my head first (not face or ears) by wearing not to warm hats.
In serious blowing snow, zip up all your pockets. I’ve had wind pack snowballs magically appear in my pockets before.
If it is wet and cold or near freezing, try your best to keep dry. If you are wet and there isn’t any precip coming down, keep moving to warm up and change out of your wet stuff ASAP. No cotton, only fast drying synthetics. If the weather is calm you can also remove your wind layers and steam the water off by being physically active.
Keep your water bottle upside down, water freezes top down. i keep a small 500mL bottle in my parka to keep topping off with snow so I can have water to drink.
Fuel gives instant frostbite at under -10F. Wear gloves.
