The CART 2002 carset may be complicated, but what you have to do is look at the Default directory as being the "base". The IRL GP3-2K and F3000 GP3-2K carsets are just that - one performance file and one set of .jam files.
You wanted an idiot's guide - well, I'm not saying that this is that, but what you can do is start with a GP3 carset in GP3Edit. Export that to your GP3 installation, then copy the helmet (software AND hardware) jams across to GP3-2K, then copy all the car and cockpit jams across, (hardware OR software, depending on how you're running it). Check out how I install them in the different directories, to avoid confusion - I also rename them, so I can work out which car is which!
Repeat this process with your alternate cars - you only need to copy the helmets once.
Save the performance file out of GP3Edit as a .txt file and move it into your GPxPatch folder.
Point GPxPatch at the relevant .jams and you should be in business. Tyres are a lot more complicated, but the same principle applies - if it works in GP3, you can get it to work in GP3-2K
Hope that helps