A substitute is simply an adorer with a special flag on their account. Learn more about adorer accounts.
To add an adorer to the substitute list, edit their account, check the Notify me of substitute requests field, and click Save.
To view the list of substitutes, visit Admin » Adorers, enable the Substitute filter, and select Is a substitute.
You can also view the substitute list on the Directory page by checking the Show only substitutes field.
To view the weekly substitute coverage, visit Admin » Substitutes.