A lot of times you have a setup and start turning laps. You don’t know what to change on the car to make it do what you want.
I found this cheat sheet online somewhere for NR2003 and the tweaks work just fine for iRacing Oval cars.
The light red are tweaks and you have from left to right, general, corner entry and finally corner exit to help you with what to tweak for what condition.
Bright Red are the big swings, and will make larger changes to how the car behaves.
The general idea is to take larger swings til you get change and then fine tune with the tweaks. Something to keep in mind is how the car will change over the course of a fuel run.
When making changes, ensure you change one thing at a time then go run 10 or so laps to see how it affects the car. If you change more than one thing at a time, you won’t know which change fixed / harmed the issue.