So, uh...how do I do them. That's a long story.
There are 3 things I need to take into account for each track.
1) What characteristics does the track have? Slow corners? Fast corners? Do I have to reach top speed on the straights? Can I make it even if I lower the top speed and increase the cornering speed instead? When should I shift?
2) How low can I go? You need to find the absolute minimum downforce and apply it in order to be consistently fast - just so much downforce that you can get around the track but quick enough to beat the "Legend" AI or human opponents online.
3) Would the setup actually cost me time by having to stop more often? Extremely fast setups can finish your tyres off really fast if you're not careful enough.
I'm often sitting in our living room with a notepad and take notes about the speed and gear for each corner as well as the top speed that I'm going to need for both the main and back straight. I'm also watching onboard laps in order to check the rhythm of real F1 drivers. To be clear on that: they're not indicating the top speed but merely the rhythm of the track. Top speed is most often way higher in the game.
There's always a base setup since you know that something might be more important on a particular track. No need to boost your top speed instead of downforce in Monaco, for example. Everything's based on logical assumptions and the rest is often trial and error. I'm currently near ~500 crashes in GP mode. Yet, I've only completed 2 Grand Prixs and all the crashes resulted from trying out setups in practice sessions.
Many tracks have similar characteristics - the Suzuka setup hasn't been created from scratch but I used my Nürburgring setup and went ahead from there. Same for the Silverstone one. However, that's not possible for every track, which is also the reason why I'm still working on the wet setup for Marina Bay / Singapore. It's actually pretty fun to create setups - especially once you know how to make things work the way you want it.
There are different ways to achieve your goals and with the huge amount of setups floating on the internet it's only natural that some setups are better or more consistent or some might even be almost the same (both setting and result wise) as another one - yet, everyone created setups according to their own style & need, so I guess every setup created is still a unique piece of work.