My general rule of thumb for exercising outside when it's cold is to dress as if it's 20 degrees warmer.
The SP guide Coach Jen shared has sine great ideas to get started but I basically wore what it recommends for the 20 to 30 F on Monday for my 5k. When the race started at 9 AM, it was -14 F with a very light wind. I wore a jacket and pants with brushed fleece on the inside; if I had worn an actual fleece, I would have been too hot. I also wear wool crew sock that are tall enough to tuck the bottoms of my pant into to help limit ankle skin exposure. I also had a layer of Aquaphor on my face and a neck gaiter.