After clicking the Canva button you will see a modal popup.
If you have an existing Canva account you can login or create one from here: https://www.canva.com/
To save images for later use, you will need to log into Canva.com, create a folder, and create your images there. They will be accessible from your Canva account in UpViral after you save.
After you have logged in, you can select a template, from the left and then edit the text or images however you like.
When you are done editing your image, just click the publish button and it will automatically save to your UpViral social image page. You can find the Canva-UpViral connection when going to 3. Sharing > and scrolling down to 2. Share Image