Well, there are a couple of ways. The first just takes a bit of learning but not difficult once you get used to it. You need to spend a bit of time looking at the plr , rfm files and you can see various settings that can be changed. So you can put your own custom list of tracks in, you can set your own points, number of opponents, and many other things. For default cars, select championship, car, settings and away you go.
For non default car mods, you must first get the cars installed and make sure they appear in your game.
Then, depending on where they appear in the menu, you must make an rfm file specifically for them.
So for example: if your mod appears in the main menu in place of the F3 cars, you need to look for the rfm file that relates to the F3, which is reiza05.rfm.
Now, backup that file and then replace it with your own reiza05.rfm with whatever settings you want for your championship. Change the FullSeasonName, the SceneOrder, and the SeasonScoringInfo and you're done.
Method two is slightly easier. Search Google for rfactor Champ R-esult by Dinix project. Look for the Christmas version (there are other versions but I find this one runs better for some reason, other versions had bugs for me). With this little prgroam, you can simply run a single race and then add it into this program. As you go along adding each race result it will show season results, driver championship, constructors points, and other info. Once you get used to it, it is an easy way of running your own season & works really well.
There is a lot to take in at first if you are immediately adding mods and running championships, but lots of people are willing to answer your questions here. It's a great game, made even better if you take the time to alter settings to your own tastes.
