You must have valid subscriptions for Salesforce and for Office 365 for Business.
Login to https://portal.office.com to get started.
Office 365 for Business login
Go to Excel Online to create a new excel workbook. If you are using Excel for the first time, use One Drive to create new Excel workbook.
Open New Blank Workbook
Go to Insert > Apps for Office. The list of Apps are displayed.
Select SmartClient for Salesforce and click Insert.
SmartClient for Salesforce App loads to the right side of the Sheet.
Click Launch once the App is loaded and approve the auth for Office365 account.
Once the auth is successful, you will be prompted to connect your Salesforce account with the App.