This achievement requires you to upgrade all 4 festival locations.
Each of the four festival locations can be upgraded to level 5. The final festival upgrade is available once you reach 25,000,000 fans.
There are a few ways you can earn fans in Horizon 3:
-Bucket List Challenges
Events such as head-to-head races and street races do not gain you any fans since they are seen as "under the table" races and not directly affiliated with the Horizon Festival.
*EDIT* Thanks to AH GamerScore for pointing out that the amount of fans you win varies on what difficulty your playing, so all of these numbers are based on "Average" AI difficulty, which is the default.
Now I cannot attest to the amount of fans you earn before completing the final showcase, but after completing it each race and earning at least 1 star in PR stunts earned me about 130,000 fans plus bonuses from my Drivatar lineup and Skills completed during the race.
My personal suggestions to doing this, if you are going to attempt to 100% the game, is the following:
1) Make sure you do every showcase event. Not sure the outcome of losing but winning them earns you about 1,000,000 fans. I know most people will do them regardless but I say this because you can get the achievement for completing the final showcase without actually having to complete it.
2) Focus on Exhibition races first. There is an achievement linked to completing one at every single route in the game and you get about 130,000-140,000 fans per race win after you've completed the final showcase, so it really adds up and they aren't time consuming.
3) While travelling between Exhibition races, try and get at least one star on any PR stunts or complete any Bucket Lists you come across. Granted you may not have the right vehicle at the time to get 3 stars, but one starring PR stunts after the final showcase gets you 130,000-140,000 fans, plus more if you manage to get two or three stars. If you get stuck on a Bucket List challenge though, just leave it and come back to it later.
4) If you race all the Exhibition races and get tired of PR Stunts and Bucket List challenges, then you can race Championships, which are a series of races. Each race nets you about 130,000-140,000 after you complete the final showcase, plus if you win the championship you get an extra 250,000 fans.
There are more than enough fans to gather out there that you do not have to complete every race, Bucket List, PR Stunt and Championship to get this. At the time of writing I only finished 58/63 Exhibition races, 6 Championships, and haven't gotten at least one star on 5 PR events.
If there's anything I'm missing from this let me know, and if there are any questions feel free to ask. Again this is just my personal experience playing the game, not a get fans quick method.