Select your project in the startup dialog.

If you're already in the IDE, from File > Open.

Importing the Project

Once you've opened the project in IntelliJ IDEA, the IDE should automatically load the project's Gradle configuration and perform the necessary setup tasks.

If you receive a notification talking about a Gradle build script, you should click the Load button:

Once the project has been imported, you should see the project's files in the project explorer, and you should be able to start developing your mod.