How do you know how to spell spanish words?

To spell the Words out, you get to know the Spanish names for the letters of the alphabet, e.g.

A - a (ah)

B - be (bay)

C - ce (say - in Latin America; thay - Spain; 'th' as in 'thnin')

and so on. (The above gives the letter (in capitals), then its Spanish name, then (in brackets) the pronunciation of that name.

To see how the Spanish equivalents for English Words are spelt, you could consult an English-Spanish dictionary; or there are translation engines (Babelfish, google Translation, etc.) on the internet.