SharePoint 2013 - Customization for site pages only, within a specific sub site

From SharePoint 2013 -If I create a publishing sub site called "Media" and I want to customize the master page for all site pages within this Media sub site, what steps must I take to complete this without changing the administrative pages (i.e. pages in site settings, design manager, etc...). They all use olso.

I want a full customization (master page editing), of just the site pages within this specific sub site.

I may have 100 site pages within this sub site, and want them all to use my custom master template. nothing else in this sub site should use this custom master template.

Thanks!