Spelling out words

Spelling out a word is usually done orally so probably the only time you will read it is in reported speech, either genuine or imagined (e.g. in fiction). There are various styles; there is no single standard one, and certainly no rule. Spaces are quite usual, and often hyphens or commas. Upper or lower case or a mixture can be used.

"How do spell your name?" John asked Cynthia. "C Y N T H I A" she said.
"How do spell your name?" John asked Cynthia. "C-Y-N-T-H-I-A" she said.
"How do spell your name?" John asked Cynthia. "C y n t h i a" she said.
"How do spell your name?" John asked Cynthia. "C, y, n, t, h, i, a" she said.

If you wish to emphasise a correct spelling you can do it various ways.

The correct spelling of my name is CONTREX.
My name is spelled thus: Contrex.

Commas are very rarely used in this manner. There's probably nothing wrong with it but I don't recall ever seeing that in print. Dashes are the preferred style.