If you are resident in France then the tax is payable by 15 June each year; if you are a non-resident European you have until 15 July; if non-resident from elsewhere you have until 1 Sept.
The declaration should be made on Form 2725 accompanied by supporting documentation.
Non residents should contact the Centre des non-résidents at Centre des impôts des Non-Résidents (CINR), 10, rue du centre, Noisy-le-Grand (93).
Their e mail address is cinr.paris@dgi.finances.gouv.
Finance & Taxation