Open Minecraft and locate your world save folder
Press Win + R, type `%appdata%.minecraftsaves`, and press Enter
Find the folder for the world you want to transfer
Copy the entire world folder
Open the Modrinth app
Select the Minecraft instance you want to use
Open the instance folder or game directory
Paste the copied world folder into the `saves` folder
If there is no `saves` folder, create one
Launch Minecraft through Modrinth
Check the singleplayer world list for the transferred world