Do a right click on the main project (one of the three projects mentioned above) → New → Other… (or press Ctrl + N). In the window, select Maven → Maven Module
Check the checkbox called "Create a simple project (skip Archetype selection)", give the project a "Module Name" and hit Next
Enter a human-readable name into the field "Name", fill in "Description" as needed
Click "Finish".
You will see the new project in the Eclipse Project Explorer. Now you can start creating or copying source files. Using the UTF-8 encoding is mandatory