With any text editor, as long as you know the rules.
Learn the words
LilyPond does take some learning. Fortunately it is well-documented. Read at least the first two links at lilypond.org/manuals.html. Before you dive into a massive score, try notating a few short tunes. At any time, I have at least a dozen 32-bar folk tunes I can’t actually play because they’re bad photocopies of bad handwriting ((If you don’t have any scrappy folk tunes that need rewriting, try hanging around at disreputable folk sessions and you’ll soon acquire some.)).