Start adding a plugin to a placeholder: open the plugin picker of the placeholder and choose the plugin type. The plugin's form opens in the CMS modal; fill it and finish with "I save the plugin".
Examples
When I add a "Text" plugin to the "Page Content" placeholder
When I add a "Link" plugin to the "Page Content" placeholder
And we add an "Alert" plugin to the "Page Content" placeholder
When add a "Card" plugin to the "Page Content" placeholder
And I add an "Accordion" plugin to the "Sidebar" placeholder
Pattern
When(/^(I |we )*add an? "([^"]*)" plugin to the "([^"]*)" placeholder$/, …)
Part of @webship-js/django-cms-steps (github.com/webship/django-cms-steps), a step pack for Webship-js 2.0.x.