To create SMS payment notifications for FreshBooks using Twilio, you'll need a Twilio account, a FreshBooks account, and PHP web hosting. You'll set up a webhook in FreshBooks to send HTTP requests whenever payments are posted, and then write a script to look up the payment and send an SMS message using Twilio. The script will also verify the callback from FreshBooks to ensure it's coming from you. Once verified, you can enter a payment to one of your FreshBooks invoices and receive a text message notification.