You will need to use Tableau Desktop to publish the datasource (excel file)
In Tableau Desktop you will need to connect to the Excel file using UNC path and not drive letters (ie \\servername\share\folder\excelfile.xls)
The 'run as' account that tableau server is running under will need to have read permission to this file / folder
When you publish the data source using Tableau desktop , specify that you dont wish to include external files.
That should do it I think
Is there any video on this?
Not sure.. I havent seen one.