Open the form of a plugin for editing, from the structure board. Name the plugin by its type; "first", "second", "third", "last" or a number picks one of several ("the second "Text" plugin", "the 4th "Text" plugin").
Examples
When I edit the "Text" plugin in the "Page Content" placeholder
When I edit the second "Text" plugin in the "Page Content" placeholder
And we edit the last "Text" plugin in the "Page Content" placeholder
When edit the first "Link" plugin in the "Page Content" placeholder
And I edit the 3rd "Text" plugin in the "Sidebar" placeholder
Pattern
When(/^(I |we )*edit the (?:(first|second|third|fourth|fifth|last|\d+)(?:st|nd|rd|th)? )?"([^"]*)" plugin in the "([^"]*)" placeholder$/, …)
Part of @webship-js/django-cms-steps (github.com/webship/django-cms-steps), a step pack for Webship-js 2.0.x.