I've just always asked...
書いてくれませんか？ (Could you please write it down for me?)
入力してくれませんか？ (Could you please type it out for me?)
この言葉をゆっくり話してください。(Please say this word slowly.)
ひらがなで書いてくれませんか？ (Could you please write it in hiragana for me?)
I agree with everyone here in that 綴る seems a bit unnatural to use for Japanese itself. I asked my one friend how they "spell" in Japanese and he looked at me like I had five heads. Haha. So, it seems more common to just sound out words slowly or just write them down.
EDIT: I found if you want the word in Roman letters, just ask...
ローマ字で書いてくれませんか？ (Could you please write it in Roomaji for me?)
I imagine if you ask them to say it in Roomaji that they'd start spelling with the A, B, C, stuff. Try it. Hah.