1. Go to https://www.messagexchange.com/authorise/supplier-einvoice.aspx 
  2. Click connect under your accounting software's logo
  3. You'll be redirected to login to Xero
  4. Complete your details
  5. You'll be shown the companies you have setup in your accounting software. Check the boxes next to the companies from your Xero account that you want to send e-invoices to. You must enter an ABN or NZBN for each company.
  6. Enter the payment details to be sent on your invoices. Either select details from your software or enter your own:
  7. Tick the box to agree to the terms and conditions
  8. Click submit

Once you're setup, each invoice you create in your accounting software for the organisations you ticked in step 5 will be sent as an e-invoice. Keep in mind that they may take an hour to send.