For me it changes with every track, sometimes I start with a melody, sometime a drumloop, sometimes a bassline, sometimes a pad etc etc. What usually happens is Ill get some sort of inspiration from a sound and then build around it, and the source of inspiration usually ends up being the centrepiece of the tune, not always though. As for mixdowns, other than levels, I tend to do most of it after I've put down the main structure, I like to get it all out before I lose it ya know. I seem to work way differently to everyone else I've ever talked too about this, but it works most of the time so meh. To me it seems a bit limiting and formulaic to start every tune the same way...