Animal Crossing New Horizons has a feature that allows you to fully customize your own island. You can place rivers, mountains and all the other essentials that makeup your dream island.
However, the tons of various creation possibilities can be a bit overwhelming if we don't have clear ideas. Changes are almost made immediately, but a tool that can help you to design your island has just been released.
This tool allows you to build your island and edit it in order to have a design that continues on New Horizons.
Island Planner is what you need for this as it gives you all the necessary tools to design and create an island and of course, allows you to work on the reliefs and terrain features.
If you are not very clear about how you want to design your island, you can always count on this useful tool that will allow you to create a sketch similar to the game through simple commands and steps.