Right at the start we pointed out that a sleeping bag will not make you warm: it will only limit the rate you lose heat. That heat comes from within you. But there are many tricks of the trade we can use to improve our comfort and warmth. Some were mentioned before, but never mind.
Every outdoors person should be as handy in the use of a map and compass as they are with a knife and fork. With a good map, and knowledge of its use, you can visualise the other side of a mountain, or what lies in the deepest valley. It is easy to proceed confidently into unknown country and get to...