In your dashboard left-hand navigation, go to TEXT MESSAGING > SEND BULK MESSAGES.
Then follow these steps:
Click on +New Text Message button at the top.
Type your message in the text box.
Choose to send to your entire list or a certain group.
If you'd like, you can preview the message.
Lastly, you can schedule the message for later delivery or send it right away by clicking the SEND NOW button.