Significance: The expansion of fNIRS methodology and analysis tools give rise to various design and analytical decisions researchers have to make. Several recent efforts have developed guidelines for preprocessing, analyzing, and reporting practices. For the planning stage of fNIRS studies, similar guidance would be desirable. Study preregistration helps researchers to transparently document study protocols before conducting the study - including materials, methods and analyses - and thus others to verify, understand, and reproduce a study. Preregistration can thus serve as a useful tool for transparent, careful and comprehensive fNIRS study design. Aim: We aim to create a guide about the design and analysis steps involved in fNIRS studies and provide a preregistration template specified for fNIRS studies.Approach: The presented preregistration guide has a strong focus on fNIRS specific requirements, and the associated template provides examples based on continuous-wave fNIRS studies conducted in humans. These can, however, be extended to other types of fNIRS studies. Results: On a step-by-step basis, we walk the fNIRS user through key methodological and analysis-related aspects central to a comprehensive fNIRS study design. These include items specific to the design of continuous-wave, task-based fNIRS studies, but also sections that are of general importance, including an in-depth elaboration on sample size planning. Conclusions: Our guide introduces these open science tools to the fNIRS community, providing researchers with an overview of key design aspects and specification recommendations for comprehensive study planning. As such it can be used as a template to preregister fNIRS studies or merely as a tool for transparent fNIRS study design.