1. Go to Office.com/setup
2. Enter your activation key
3. It will then ask you to set up a Microsoft Account
4. Create an account based on the computer's serial number.  The email should end in @outlook.com  EX. MXL204782U@outlook.com.  If the SN starts w/ a digit, add a prefix such as SMG_726BLX574@outlook.com (for Sampson Morris).  
5. Document the Office install.
6. Make sure you document the Online product key by clicking "View your product key.
7. Confirm Office is activated by opening Word, Excel, Outlook, etc and go to File - Account.  It should say activated.

If you need to re-install Office at any point and the account is already set up, just go to login.microsoftonline.com and login with the credentials.  Click "Install Office" on the front page.  There will now be a "Install" in orange on that page.  Click it and install Office.