When studying the assessment of diet quality in Brazil, we identified the need to develop a new index that should be easy-to-apply, easy-to-analyze, and understandable, and to provide a more accurate measure too. In this sense, we investigated analytical methodology to obtain a measure with better psychometric proprieties and less demand to answer and analyze. To investigate the psychometric characteristics, it is necessary to gather five evidences of the validity of the questionnaire, including the study of the test content, response process, internal structure test, consequences of using the test, and association between test results and other variables. In this case study, we shared with you our experience to study the test content using a qualitative approach and the internal structure test using Item Response Theory. We invited experts in nutrition to discuss the relevance of the items in the questionnaire on focus groups. We analyzed the data using the exploratory content analysis that provided in-depth reflections of the opinions. After considering the suggestions of the experts, we shared the questionnaire in social media. When analyzing the answers, we studied the quality of items in the assessment of diet quality using Item Response Theory. These analyses also provided a description of the eating practices around the different levels of diet quality. Now, we invite you to read this research method. We are sharing our experience that may stimulate you to use the methods with potential to improve the development and accuracy of different kinds of measures.