Yeah usually it's whatever i'm inspired to do first. I find when i write the lyrics first before the song, i won't like them and wont end up using them... I'll usually be stuffing round on my guitar and bash out something that sounds half decent. I try to remember what i played and organise them into some kind of form and just play over and over. Once i know what i'm playing... i'll start singin words that come to my head as i'm playing, just completely on the spot. Phrases i blurt out that i like i'll write down. I'll take it from there, see what i churn out and polish it up a lil... that's my song. It seems to be working for me, but not everyone's all that good on improvisation, i know i never used to be!!