There are two ways in which you can create a template.
#1 Create a template from scratch
Go to Settings > Templates and click Add a Template
Give your template a name that will allow you to remember the template's content.
Compose your message as you would any other message (yes, you can personalise your message with custom fields too). The counter at the bottom of the box will give you an indication of the message length (remember, if you use personalised fields the count will be approximate).
Save your message and your template will be created.
#2 Create a template while creating a campaign (save a template)
While creating a new SMS campaign, you'll hit Step #3 of the wizard: Compose your message.
Select Start from scratch and go about it as you always do, but at the end check the Save as Template box, assign it a name, and voila!
Your message is now saved as a template, ready to use whenever you need it!