In Animal Crossing: New Horizons, there are two cool things you could do – building paths and terraforming (landscaping and waterscape) your island.
These powerful features are only unlocked later in the game.
In order to unlock the Island Designer App, you’ll first have to complete the main campaign.
That would mean having at least 8 villagers, building facilities such as the Town Hall, Museum, Nook’s Cranny, Able Sisters Tailor Shop, and the Campsite.
After that, you’ll unlock something called Island Evaluation, which is graded between 1 to 3 stars.
You can find out your rating by talking to Isabelle at the Town Hall.
To get 1 stars to 2 stars, you’ll need to have 8 villagers on your island, and to get from 2 stars to 3 stars, you’ll have to fully decorate your island with flowers, trees, and crafted furniture.