First, click on the 'Use Template' link to the right.
On the next page, click 'Use Template' again.
Once the template is opened, give the design a Name, so that it's saved in your Canva account.
Now, open a new tab, and log into thanks.io. Navigate to your Image Templates, and click on the 'Design Postcard' button at the top right of the postcards section.
Click on 'Folders' on the left of the Canva designer.
Then, 'All Your Designs.'
And your template should be visible. Click on it to load a copy to edit!
When you are done editing, be sure to click 'Save' at the top right and it will be added to your image templates.
You can create as many different designs off of one template this way as you need.