Hop in a fast car (not one that runs perfect on dirt!). It doesn't have to be R1 or anything, just the fastest car that is available to you that you can handle off-road.
Head to the golf course located southwest of the Festival Hub, in the northeast corner of the Montano Plains. You can fast travel to the Media Center Outpost and head east to be there quick. In this golf course, there is a big plateau feature that has one steep ramp shaped side and another gently sloping side. On the steep side, there are several smaller hills and sand traps which can be used to gain big air. Run your car throughout the golf course, bouncing over little hills (get your car to bounce up a few times to get the Kangaroo skill worth 1000 pts.), and flying up and over the plateau. You can chain drift, speed, air, Kangaroo, and Crash Landing skills together to increase your multiplier and rack up major points fast. Keep going until you hit Popularity 100!
You can also choose to just drive and race and rack up popularity points throughout your time in Horizon. If you're just driving around, try to score Ultimate Wreckages (hit 8 destructible objects in quick succession) for 1000 pts. and go for 3 near misses in a row to hit the Daredevil skill for another 1000 pts. plus the pts. for the near misses. Drive between two cars that are passing each other or in adjacent lanes on the interstate to get 1000 pts. for Threading the Needle.
You can buy the Popularity Spike through the marketplace to get 2hrs in-game double popularity to make it even faster, but with this method you won't need to worry about that.