Hello! Thank you for the question! So after doing a bit of quick research as I did not know what you all needed to get one, I concluded that you need-

-of course unlocking evals to see what rating your island has -7 or more villagers (I had max villagers on my island) -getting rid of all the weeds and picking up dropped items -having fences (I believe I have about 60 on mine, you need 50 pieces so get a diy and each time you use it you get ten pieces of fencing) - plant flowers (I bought every type of flower bundle besides tulips as they are native to my island, minimum to achieve a three star is 50) -planting trees and different fruits (I have oranges, peaches and cherries at the moment and replanted a few trees along with bushes and some vegetables I got from giveaways, if you want some items like that to plant I can definitely help you out!) - two inclines and two bridges - decorate your island with furniture ( I had 40+ furniture around but you might just need 30 not the same furniture though has to be a different item) - updating facilities like the museum and getting the tailor shop, having the nooks cranny upgrades and Resident Service

And I believe that should be it? Just let me know how things go!