How to stay warm on the coldest days

With the recent cold snap here in Canada, it’s been fairly unbearable to be outside at all. I am still outside ski instructing and have previously been camping in temperatures similar to this a couple winters ago. My mom was headed out on the slopes last week and asked how do I dress in order to stay warm tomorrow? So I figured I would write a post explaining what I told her. So really, how do you stay warm when it’s -27 and you have to be outside all day?