You'll need to sign up for a free Xero account and add a OAuth 2.0 Xero App. You can view Xero's getting started guide (opens new window) for details on this.
Once your Xero app has been created, enter the Client ID and Secret from your Xero app in Xero's plugin settings.
Navigate to Organisations to create an Organisation, and save it. Click the Connect button, and proceed to authorise with Xero, selecting the organisation you wish to link in Xero.
Create multiple organisations as required, every enabled organisation will be used to send data to.
Once an organisation is connected the plugin allows you to map your Chart of Accounts (Account Codes) including:
By default, all fully paid orders will be pushed into the Craft Queue with a delay of 30 seconds, after which the invoice will be sent to Xero.
You can also send existing orders to Xero when editing an order, and clicking the Send to Xero button.