3 Weekly Community
Events. You don't need to worry about which tier you finish in since you simply have to reach the end of every stage in the event. Additionally, make sure you complete community events and not weekly AI challenges since those don't count.
You will have to finish around 5-10 stages to complete one event, but these don't have to be done in one sitting. You can quit to the menu at the end of a stage if you need to and your progress will be saved the next time you come back. Make sure you do these in a timely manner, however, since weekly events reset an hour or two before noon GMT on Tuesdays. On the flip side, Tuesdays are when you should look out for a new event.
To access Community Events, select "Events" from the front page. Look in the top right section of your screen and you will see some small icons telling you which category of events you are currently in. Hit
until you reach the one labeled "Community Events." Now, scroll a couple more tiles until you find the Weekly challenge. Make sure you have the right car and you can get started.
EDIT: Per the comment from NegativeCreep08, you will have an easier time with this if you upgrade your co-driver so you can recover from a crash that would otherwise end your run.