Reference: Convert a regular project into a maven project

1. Open the existing project in IntelliJ
2. In the Project tool window, right-click your project and select Add Framework Support.
3. In the dialog that opens, select Maven from the options on the left and click OK.
4. Open the generated POM and specify a groupId. The artifactId and version are specified automatically.
5. Add more depcendencies by updating pom.xml.

For instance, add Junit5 support:

1. Open project level pom.xml
2. Press Alt+Insert. Select In the dialog that opens, enter junit and select org.junit.jupiter:junit-jupiter.
3. Click Load Maven Changes in the top right corner.