How to fill out a Déclaration des revenus

Once you establish residency in France, you are taxed here on your world-wide income. Under French law, you are a resident in France for tax purposes if you meet any one of the following four conditions:

Your permanent home (habitual home for you and/or your family) is in France. You spend most of your time in France (at least 183 days during a calendar year, or even less if you spend more time in France than in any other country). Your professional activity is in France. The centre of your economic or financial interest is in France.

via www.expatica.com