Before exporting invoices to Xero you will need to setup Inventory Items in Xero, please see the Creating Inventory Items article for more information.
1. Ensure that the customer is linked with the accounting system  (their name must be selected in Acc. Dropdown list. If the customer is not in the list you will need to export them first, see the article on Exporting Customers for more information.
Then ensure that each line has an Acc. Item  and Tax Status  selected. The default for the Tax Status is GST unless otherwise selected.
If required you can enter information in the 'Comment' field below the grid. This field is exported to the 'Reference' field in Xero. This information can be helpful when sending statements form Xero as it is listed beside the invoice number on the statement making it easier for your customers to understand what each invoice in a statement is related to.
Finally click Export 
2. You can verify the export by checking the Acc. Total field  and if required check the status between the two systems by clicking the E  button to update this total.
3. If required view the invoice in Xero.