Better really to make up the tune and sing the song a few times before reaching for the guitar to work out an accompaniment (if you decide it needs one)..

Agree totally, McGrath of Harlow. If I try to use the guitar to work out a tune, it inevitably restricts the outcome (maybe that's a reflection on my limited ability, however). I prefer to think of a tune in my head first, and then work out the arrangement as a second stage.