Begin with a comfortable base
Choose an inner piece that feels right on its own. Then check whether the next layer has enough room at the shoulders and arms for movement.
Separate warmth from bulk
The number of layers alone does not tell you how warm an outfit will be. Use confirmed fabric details and your own experience; avoid assigning a temperature rating to an untested item.
Watch the lengths
Let one hem remain visible or keep a clean line, depending on the effect you want. Try the full outfit before deciding, especially when a longer outer layer meets a wide trouser.