Add a section with one of the layouts the site offers ("One column", "Two column", "Three column", "Four column" come with core) at the end of the layout on screen: the last "Add section" link, then the layout in the off-canvas dialog. Core opens the section's configuration form next; "I save the section" confirms it.
Examples
When I add a "Two column" section at the end of the layout
And I add a "One column" section at the end of the layout
When we add a "Three column" section at the end of the layout
And I add a "Four column" section at the end of the layout
When I add a "Two column" section to the end of the layout
Pattern
When(/^(I |we )*add an? "([^"]*)" section (?:at|to) the end of the layout$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.