The game tells you how many ranked series you need to complete before getting a ranking. Should be the first thing you see when you click on Ranked adventure and it gives you the option between Team Racing, Playground, and Free For All.
As an aside, the new RTR Spec 5 Mustang has an option to swap in the 7.2L Roush-Yates V8 that the RTR drift cars use. Makes 1000HP, revs to ~9500 RPM and screams like a banshee. And if
that wasn't enough, you can Turbo/Super/Procharge it to roughly 1700 HP. You can actually do all that, give AWD, Race tires with the widest possible tread, front and rear wing options, and it'll stay in S2 with some room to spare. Maxxing it brings it into X-Class.
As for the playlist system, I'm not a fan overall. I
really like that it gives all the seasonal events an challenges in an actually organized fashion, makes it easy to see what's left to do. The problem is that in order to get 100%, you not only have to play the Trial and PG games with randos, but you have to win. I don't know about anyone else, but I have the worst luck when it comes to these modes. In attempting to earn a second Hoonigan RS200, I've lost the PG games event 4 times in a row. That's super frustrating when I'm trying to do everything I can to win, but my teammates seemingly aren't up to the task. It makes the whole thing seem like a chore, especially considering that you have to 100% each season in order to get arguably the rarest car in the game. Fun times ahead.