See:

• Customize the Office Ribbon (It doesn't take rocket science) by Greg Maxey This is a Windows page and is good for background anyway. For the Mac, see
• OOXML Hacking: An Introduction by John Korchok

While you can do this on a Mac, it is a lot easier in Windows. This is because there does not seem to be a good ribbon editor for the Mac. If you have access to a Windows version, you can design Ribbon tabs using Windows that will work on the Mac. These get stored in the templates with the macros.

Consider putting your macros in custom QAT modifications in the template rather than a custom ribbon tab. This is far easier, at least in Windows. Modifying the Quick Access Toolbar (QAT) in Microsoft Word