Abstract-Today, GUI plug-ins development is typically done in a very ad-hoc way, where developers dive directly into implementation. Without any prior analysis and design, plugins are often flaky, unreliable, difficult to maintain and extend with new functionality, and have inconsistent user interfaces. This paper addresses these problems by describing a rigorous methodology for analyzing and designing plug-ins. The methodology is grounded in the Extended Business Object Notation (EBON) and covers informal analysis and design of features, GUI, actions, and scenarios, formal architecture design, including behavioral semantics, and validation. The methodology is illustrated via a case study whose focus is an Eclipse environment for the RAISE formal method's tool suite.