Like an earlier poster, I start from "What you do, you know".
When taking on a new song I like to start with a complete set of words -- before I had a decent library (and long before internet sources!), this might involve sitting with a recording and scribbling furiously, concentrating on significant words or ideas; then listening again and again and filling in the gaps; then writing out a tidy copy and doing a final check for any mistakes. Then I'd sing it from the page, repeating each verse until I could comfortably look away from the sheet and building up the song by adding a verse at a time to those that were already there. And when the whole thing was in my head I'd sing it through, or chant the words when driving, so that the rhythms and phrasing settled. And eventually I'd sing it out, preferably to an audience I knew.
And when reviving an old song from the repertoire, the process was almost the reverse -- start with the complete text and strip it down to the meaningful words/phrases that fall on the pulses of the rhythm, to reduce it to a skeleton. And then all the connections would become almost inevitable.
And even though I can now type and print, I will still prefer the act of writing, much like the lady who moved her finger for invisible shorthand in an earlier post. (Something surfaces from dim recesses -- is this eidetic memory?)
But probably the biggest impetus is to really like the words (not just the tune) and want to do them justice.