In French, to ask 'how do I spell it?' you say: Comment ça s'écrit? To answer, you can simply: a) give them the spelling of the word or b) say: C'est s'écrit ... Literally meaning 'It is...
French is "français" (feminine: française; plurals: français/ françaises). Here the word français is an adjective, so use a lower-case f. A Frenchman is 'un français', a Frenchwoman is 'une...