With a Global Layout, there are no Conditions. This is because it’s global, and any Layout Sections added to it will be used on every page. If, however, you wanted a Custom Header just on some pages of your site, you’d need to add a Header Layout Section to a Conditional Layout.
With any Conditional Layout, you need to set the conditions of the layout. This is best done last, because as soon as you set the conditions, the layout is active, and if you do this after first creating the Layout Section, but before editing it, the pages displayed that fit the conditions would be empty in those sections.
To add a condition, or conditions, simply mouse over the bottom of the Layout, and choose Add A Condition. This brings up the Layout Conditions Dialog. Please see Understanding Conditional Layouts for more information on Conditional Layouts.
In the example of the Pet Supplies Website, there is also a Conditional Custom Header, which only displays on one page, the Alternate Home Page. See below for the Conditions set to achieve that, and further below for an example of what that Header looks like on the Alternative Home page.