An Excel template file is nothing more than another Excel file with the headings and formatting that you want to specify for your file that you're creating in GoAnywhere Director. i.e. there is no specific requirement for a template file, it can be any Excel file you choose.
There is no need to add it to any of the GoAnywhere folders, but it does need to be in a folder located on the same system in which GoAnywhere is running. Then you just reference it from the Write Excel file task.