Ensure you are clear about how the two subject lenses are differ. You must use the theoretical frameworks from each subject. You will need to be an expert on both subjects and the global issue under consideration. Ensure that your analysis of data is equally balanced between both subjects. Data will often be secondary but in some cases you may be able to collect some primary data, such as questionnaires or interviews. You must consider whether the data you are collecting is relevant to both subject lenses.
In the introduction you must explain clearly why the interdisciplinary approach is necessary, and state how your local issue allows you to explore the global issues. Make it clear throughout that the only way to approach this issues is an interdisciplinary analysis. Throughout the essay you must use analysis from both disciplines in order to draw conclusions. The local case study should be a clear reflection of the global analysis, and these links should be highlighted throughout the essay.