Thank you both so much for your help!

I've read through the documentation. But, I'm still clearly missing something. Please bear with me.

I've created a new template. I add it to my document through the Template and Add-ins dialog box with Automatically update document styles unchecked. But, if I do that, I can't 'see' my new styles to pick them. Where do I look for them?

Or

I've created a new template. I add it to my document through the Template and Add-ins dialog box with Automatically update document styles checked. Then, I can find and apply my new styles. But, if I uncheck the Automatically update document styles, I receive an error saying This document template does not exist.

What have I missed? Thank you again for your assistance.