Basic tips…
Pick a “home” track to tune on, it’s important to be very familiar with it so you can focus on how the car reacts to adjustment. I like Catalunya full as it’s generic enough, but I always follow up with something like Brands Hatch or Sonoma to getting a better feel for stability at higher corner speeds. IMO the best cars to choose to help develop a feel for what does what are cars with a 50:50 weight distribution, with a front engine and rear drive layout; same tire width front and rear helps, but don’t get hung up on it.
Do a bunch of laps in the car stripped of upgrades to get a feel for how it naturally behaves. If you want to feel some contrast in base handling characteristics drive something like a Lotus Evora then compare it to a 370Z; the Z is significantly more stable and likes to drift on power, while the Evora spins like a top with trailing throttle then buttons down nicely when on the throttle.
When I intend to spend a lot of time in a particular car I’ll build it out, and then remove the race suspension (I like sport, if available), remove aero, and run the stock differential. This leaves me with adjustments for tires, swaybars, and brakes. At this point I focus on how responsive the car is to yaw adjustments at mid corner with trailing throttle. If the nose will not tuck in, I remove front bar or add rear.
Next I add the race suspension. Using telemetry I make sure camber is in the right ballpark. Then check ride height, spring stiffness, and bump (dampers); the initial goal is to get the car as low as possible with as little bump dampening as possible. After that, I’ll tweak the front to rear spring rate ratio depending on how the car drives out of corners and last, fine tune rebound dampening.
Regarding toe and caster: both can be adjusted to good effect, but it’s really easy to ruin a car with these. Some high powered RWD cars love a bit of toe in at the rear, some AWD/FWD cars love to out up front, read the description the game provides and experiment. Caster is a tricky one, and is great to increase if you have a car that wants to rotate too enthusiastically, but the downside is that it blunts turn in response; use with care.
Next add aero. At this point the hardest work is done. Add front aero for more front grip, reduce rear aero for more relative front grip. Do the opposite if you want to shift the balance to having more grip at the rear.
Last, add the race differential. This is absolutely the least understood part, generally speaking. It doesn’t help that the game doesn’t firmly establish if the adjustment sets the lock ratio or, as it seems to say, sets the speed at which it locks. The decel setting is easy, lower number for more rotation when off the throttle (braking, turn-in, mid corner trailing throttle adjustments), or a higher number for more stability (the car will be more sluggish and less willing to turn).
The accel setting is tricky. Best I can explain it is… lower setting for more stability, and higher setting for less stability. This is where knowing the car and knowing the track matters, because this setting is going to be felt mostly in how the car feels at part throttle. The higher the setting the more traction you’ll have available to accelerate, but the car will be faster to spin because you’re asking more of the outside tire because you’re telling the differential to send more torque to it faster when you increase the setting. Again, like the other adjustments, experiment.
I don’t share many tunes, but if you want to try an example of what I feel is a well sorted car I have a Honda S2000 out there that I think is very well balanced. But there are also a great number of excellent, much better than me tuners on these forums that are worth trying out as well, if for no reason other than to see what can be done with any particular car.