To send an SMS message with C# using the Twilio API, you need to install the Twilio helper library for .NET and initialize the Twilio REST Client with your Account SID and Auth Token. You then need to provide the sender's and recipient's phone numbers and the body of the message to be sent. The MessageResource.Create method is used to send a text message, which can be done in just 30 seconds once you have set up the necessary configuration.