Lots of good suggestions.
I’ll emphasize that feet must stay dry though! For me that means I don’t turn the floor heat on in my car on the way to the barn. I’d rather get there with dry chilly feet that will warm up once I start walking around, vs warm wet feet that will be frozen eventually.
I can usually tough it out in my regular tall boots until -10C (15f), colder than that I pull out my winter paddock boots and full chaps. I don’t ride much below -15C (5f) though since it’s hard on the lungs.
I find that my hands usually stay warm as long as my core is warm. I wear bigger mittens, then switch into winter riding gloves with a thinsulate lining. I like the gloves since I can keep them on for doing up the girth, and noseband, etc. I have some of the winter riding mittens (where the pinky is separate), but I find I have to take them off so many times when I’m tacking up that my hands just end up colder.