Final answer:
After selecting the File tab, Simon should select 'New' to access the invoice templates available on Office.com, then choose and open the preferred template.
Step-by-step explanation:
When using an invoice template from Office.com, the first step after selecting the File tab is to select 'New'. This will allow Simon to access a variety of templates, including invoices. After selecting 'New', he can then search for invoice templates using the search bar, or select from the suggested templates available. Once the desired template is found, he can click on it to create a new document using that template.