Acquire a pair of calf- or knee-length boots, any colour is fine. Tuck your jeans into them. Now put on a plain, generic t-shirt. Anything will do - no-one will ever see your bottommost layer unless you're in the throes of passion in a particularly well-heated room. Possibly not even then, I haven't run all the appropriate tests. Add a cardigan, preferably grey. Now a heavy winter coat. Top with a chunky scarf looped in some sort of an artlessly stylish fashion that I can't replicate even though I'm pretty sure I loop my scarf along the same basic principles. If the temperature has dropped below the 'critical' zone of 15 degrees, you are entirely justified in adding a knitted hat and/or gloves to complete the ensemble.
The French boy's guide to winter dressing:
As above, except substitute shoes for boots and don't tuck your jeans into them. Ditch the cardigan - you, sir, are manly enough to face the cold with only a peacoat and a chunky scarf, also cunningly looped. If you want a bit of Fonz-esque je ne sais quoi, you may choose a leather jacket. For rebel chic, go with a ski jacket and beanie, and lose the scarf (too bourgeois). If at all possible, add a jaunty messenger bag to your outfit. A headband is an optional - but stylish - extra.
The old French man's guide to winter dressing:
Put on anything and add a flat cap and a querulous stare. Congratulations, you are now an old French man.
The old French woman's guide to winter dressing:
Put on weight. You starved yourself for 60 years, you deserve this.