You need to be analitical in your approach, start by asking yourself, what is the problem, then describe the problem in detail. From there you can then make changes apropriate for the situation. Start with the biggest issue first and then do as the poster above me said and make one change at a time and see how that works out, paying attention to if it makes it better or worse.
I generally look at my lap and find the places where i am loosing time, on the straights, fast sweepers, tight sections, on the turn in, on the drive out, on the changes of direction, on breaking, on power. Then look at that one point in detail, whats the car doing there which it could be doing better.
Say I am lacking rear grip in the middle of a fast sweeper, i go to my trusty list of settings and the effect thay have on the car and find ones that are going to effect rear grip. If there are a couple of things that effect what i am trying to fix, i will start with the one that has the most effect on that and make a change, test, make more changes and test again.