Framework-specific modeling languages (FSMLs) help developers build applications based on object-oriented frameworks. FSMLs model abstractions and rules of application programming interfaces (APIs) exposed by frameworks and can express models of how applications use APIs. Such models aid developers in understanding, creating, and evolving application code. We present four exemplar FSMLs and a method for engineering new FSMLs. The method was created postmortem by generalizing the experience of building the exemplars and by specializing existing approaches to domain analysis, software development, and quality evaluation of models and languages. The method is driven by the use cases that the FSML under development should support and the evaluation of the constructed FSML is guided by two existing quality frameworks. The method description provides concrete examples for the engineering steps, outcomes, and challenges. It also provides strategies for making engineering decisions. Our work offers a concrete example of software language engineering and its benefits. FSMLs capture existing domain knowledge in language form and support application code understanding through reverse engineering, application code creation through forward engineering, and application code evolution through round-trip engineering.