Good to see you picked up some pants rated at 15/10.
Now for layering under those shells...
If you do a search here for "layering" or "base layers" you'll find a lot of information.
Brands like Patagonia, UnderArmour, Icebreakers, etc and most snowboarding brands like Airblaster, Dakine, Burton, etc offer base layers.
The shop you purchased the outerwear from might have base layers you can check out?
Best thing is to get layers with wicking properties. Stay away from materials such as cotton - cotton retains moisture so when you sweat, you'll be wet then cold.
Staying dry under your layers is key to keeping warm.
I prefer shells as outerwear. Depending on the temps - if it's really cold, I'll do a base layer top and bottom, a technical fleece on top, and something like a fleece or yoga pant, then my shells. But as I said, everyone's body runs temp differently. You really have to figure out what works best for you and consider the temps you'll be in.
Layering is ideal because you can always shed or add layers as needed on-hill. Versatile.
I like neck gaiters/face masks, too, especially for those wickedly windy days.
Put something on your head also. A helmet really does keep the heat in. But if you choose not to wear a helmet, get a warm hat.
Spend some time looking around the forum after doing a search to get some ideas, then check out your shop. Try some things on, ask the staff, see what feels comfortable to you. Brands and styles of layers I, or others, like may not be what you find works for you.