This documentation is intentionally elementary. Instead you should review the knowledgebase.
Note: This library is a child theme of twenty fourteen, just in case you want to download the entire library and try out customizing sprout invoice templates.
Example templates are available within this library to show files can be overwritten by a (child) theme. It's highly recommended that you copy a fresh version of each template you want to customize (don't copy over any templates you don't want to customize within sprout-invoices-pro/views/templates/*).
Multiple Custom Templates
Review the invoice-esp.php file within this directory as an example. The selection will be available on the invoice information area.