Salesforce to Xero Callouts - Authentication Setup

I need someone to walk me through how to setup a Named Credential in Salesforce that will allow me to connect with the Xero API. I have a dev org and a Xero trial account.

The work will be done through screenshare using Zoom or Skype.

At the end of the exercise I want to be able to do something like this -

HttpRequest req = new HttpRequest();
req.setEndpoint('callout:My_Xero_Trial/Invoices');
req.setMethod('GET');
Http http = new Http();
HTTPResponse res = http.send(req);
System.debug(res.getBody());