Install like any other addon. If this is your first addon, please follow these instructions: https://www.eq2interface.com/forums/faq.php?faq=howto_install#faq_new_faq_item

Copy which ever version of _ShowHelmet.xml to your addon UI folder and add the EQ2UI_custom.xml include line to your EQ2UI_custom.xml in your addon UI folder .

You can replace the _ShowHelmet.xml with on from the optional folder if you prefer one of thosee styles. The custom.xml include does not need to be changed to use them.