To build an SMS reminder service using Python and Twilio, create a new project directory, install dependencies, and define helper functions for reading and writing reminders to a JSON file. Implement API endpoints for creating, getting, and deleting reminders, as well as sending reminders due today. Set up a Twilio account, obtain credentials, and configure the service to use these credentials. Deploy the application to PythonAnywhere by committing to a GitHub repository, creating a virtual environment, installing dependencies, loading environment variables, and configuring WSGI settings. Finally, schedule the `send_reminders.py` file to run daily using PythonAnywhere's task scheduling feature, ensuring that reminders are sent regularly.