Already have some Elm code that you want to edit in IntelliJ? First, make sure that you have the Elm plugin installed. When you open IntelliJ, you'll see a launch screen.
You might be tempted to click "Import Project" but there's a simpler way. Instead, click the "Open" button and select your existing project directory.
Once your project loads and you open a .elm file, you will be prompted to setup the toolchain (paths to things like the Elm compiler as well as elm-format and elm-test). After that's configured, you'll be prompted to attach an elm.json file. Use the file picker to locate the elm.json file for your project.