How do you spellsign as in sign a paper?

To "spell sign" means to write out your name letter by letter as a signature, often used to clarify how to sign your name for official purposes. For example, if your name is "John Doe," you would write it out as "J-O-H-N D-O-E." This method is sometimes used in situations where a person's signature may be difficult to read or for formal documentation.