This article presents the process of constructing and validating a test of metalinguistic awareness (MLA) for young school children (age 8-10). The test was developed between 2021 and 2023 as part of the MetaLearn research project, financed by The Research Council of Norway. The research team defines MLA as using metalinguistic knowledge at a point in real time. To develop the test, the research team relied heavily on general measurement theory (psychometrics) and insights from the field of language testing and assessment. This aids in the process of developing a practical and user-friendly MLA-test which yields valid scores and reliably measures the state and the development of MLA. This innovative approach to MLA testing offers explicitness about the theoretical construct of the test and at the same time contributes to controlling the difficulty and discriminatory power of each test item. We argue that the test development process presented here is useful when developing tests of MLA and similar concepts, as it facilitates comparison of results as well as replicability. This development process and its detailed description also make it possible to adapt the "MetaLearn MLA-test" to other contexts.