Because of the way the speech engine works, it's best to do this using commands for the alphabet to improve recognition(though it still won't be perfect for such short commands).
Attached is a profile with 3 commands:
"spell"
Enables speaking the NATO alphabet for text input; If spoken while spelling is active, it will press enter, so you don't have to end and restart spelling for each line.
"[Shift ;][Alfa;Bravo;Charlie;Delta;Echo;Foxtrot;Golf;Hotel;India;Juliett;Kilo;Lima;Mike;November;Oscar;Papa;Quebec;Romeo;Sierra;Tango;Uniform;Victor;Whiskey;Xray;Yankee;Zulu];One;Two;Three;Four;Five;Six;Seven;Eight;Nine;Zero"
Types the actual letters. I chose this as an alternative for 36 seperate commands, because you'll be able to change the keypress delay easily for all characters within the command(Bah, you can't do it for all keystrokes at once, sorry), and it's less command list clutter; You can also speak "Shift" before a letter to make it a capital letter.
"endspell"
Presses enter, then disables speaking the NATO alphabet for text input.