In the Content section select New SMS message.
Once the message creator opens, enter a message name (this is for you to identify the message, customers will not see this)
Once all this information is correct you need to click the Message Content tab to open the SMS editor.
This is where you can enter in your text message.
You can add personalisation to your message from the personalisation tab just below the text creator box at the bottom.
Once you have completed your SMS you can click Test at the top of the page to send yourself a test alternatively you can click Finish to save the message.