Once you have connected your Xero account, to post an invoice to Xero, view the invoice in question.
Click on the orange tab at the top right then click on the "Post to Xero" button. If this invoice has already been posted to Xero, the tab will be green and the button will say "Update in Xero". There will be another button titled "View in Xero", and when clicked, will take you to that invoice in Xero.
Note: When you post an invoice to Xero, it will be in a "DRAFT" status, so you need to "Approve" them in Xero.