Some great advice that Yoeri Gijsen gave me in the ISI forums:
1) Pick a track
It’s probably good to pick a track from the calendar – preferably one that has both fast and slow turns, both long and short turns. For example I like to test cars at Silverstone or the Nürburgring.
2) Learn the track
If you don’t know the way around the track yet, focus on that first. You’ll want your test to be solid, so you don’t want to learn the track as you go through the cars. If you would, you’d find that you’re slowest with the first car you test and fastest with the last one, so don’t. Learn the track first, so you know how to drive around it fast and consistent.
3) Test each within a limited timeframe
Next you’ll want to take the cars one by one. Give yourself like 5 laps to get the feel of car and then 5 laps in which you push the car to the limit. Done? Next car!
4) Pick the car
In the end you’ll know which car is fastest and which car feels best. If they are the same car it’s an easy pick. If they are a different car, then you have a choice to make. If you’re going competitive, you might want to get over the feels and set yourself to master the car which you were faster in. If you’re driving for fun, I’d go with the car that feels best.
5) Create a personal baseline setup
Next is the finetuning. You’ll want to get the most out of the car, so you’ll have to go set it up. Stick with your car-of-choice and the track you tested on and start working on a setup. Find out what the setting do to the car and if the car goes around the track faster or feels better by the changes you make. Once you go a baseline setup down, you can simply take it to other tracks and make minor adjustments for those.
Keep in mind the REAL ROAD
It’s probably a good idea to get a whole bunch of AI go over the track first using Time Acceleration, so there is a somewhat reliable grip level. Then save the REAL ROAD and reload it with every new car.