Open a template from the Templates panel of the editor, by the name the panel shows: a content template ("Full content template", under its content type) or a page template ("Mercury theme"). The editor then shows the template instead of a page, with the template's name in the top bar.
Examples
When I open the "Full content template" template in the Canvas editor
And I open the "Mercury theme" template in the Canvas editor
When we open the "Full content template" template in the Canvas editor
And we open the "Teaser template" template in the Canvas editor
Given I open the "Full content template" template in the Canvas editor
Pattern
When(/^(I |we )*open the "([^"]*)" template in the Canvas editor$/, …)
Part of @webship-js/drupal-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.