To use Canva create a graphic design with custom dimensions, log into your account then select "custom dimensions" in the upper, right corner of the screen.
Whenever I write about Canva I get emails from teachers who want to know how students under age 13 can use it. The short answer is that Canva supports using G Suite single sign-on which means that students can use their school-issued Google accounts to use Canva even if they don't have active email addresses. The other option is to use a temporary email address service like those listed by Larry Ferlazzo to create accounts.
This video that I made will show you how to use Canva's webpage design and publishing tool.