For something quick I start with tyre pressures and gear box (if permitted), and then I go about trying to dial out excessive behaviours with aero, diff and suspension settings in no particular order. I use Chris Haye's cheat sheet to see what to change in any given scenario.
For a thorough setup I have this old step by step guide from GTR2 that I follow, which really helps me as in all my years of simracing I never really learned enough about setups to be able to do those in depth settings freehand.