There is increasing interest in health professions education (HPE) in applying argument-based validity approaches, such as Kane's, to assessment design. The critical first step in employing Kane's approach is to specify the interpretation-use argument (IUA). However, in the HPE literature, this step is often poorly articulated. This article provides guidance on developing the IUA using a worked example involving a workplace performance assessment tool. In developing the IUA, we have drawn inspiration from approaches used in the discipline of language assessment to situate the inferences, warrants and assumptions in the context of the assessment tool. The worked example makes use of Toulmin's model of informal logic/argumentation as a framework to structure the IUA and presents Toulmin diagrams for each inference such that the reader can connect the argument chain together. We also present several lessons learned so the reader can understand the issues we grappled with in developing the IUA. A well laid out IUA allows the argument to be critiqued by others and provides a framework to guide collection of validity evidence, and therefore is an essential ingredient in the work of assessment design and validation.