i would start off with asking him for the exact requirements for the plugin.
Then do a simple domain model identifying the real-life elements involved, like for eg. the browser, etc.
Then do an interaction diagram that identifes how the plugin should interface with different systems, followed by a class level design for the plugin components.
Then ofcourse develop the use cases, the collaboration diagrams and sequence diagrams from within a tool like
ROSE.
i guess what he meant by the question was to see if we could walk through the entire steps necessary for solving the plugin creation problem.