1. Log into your MyGuestlist account
2. Click on the SMS tab
3. Click on Manage Templates (located in the orange menu, top left in the window)
4. In the first drop down, select Create a new template
5. In the second drop down, select SMS
Now you just need to enter the template name and enter the text you would like to include in the message.
You can also see how many characters you've used as well as how many messages this template would be sent in.