How do you spell in Greek?

The Greek alphabet is used to spell in Greek. It's not a direct sound-to-letter correspondence like English, and the pronunciation of some letters has changed over time (Ancient vs. Modern Greek). There's also a difference between spelling a word and writing it out phonetically.

To "spell" in Greek requires knowing the Greek alphabet and the spelling conventions for the specific word. You can't simply translate English sounds into Greek letters and expect it to be correct. You'd need a Greek dictionary or a transliteration system (which often isn't perfect).