The Internet has become the main information source of graduates, trainees and young professionals, who use online information to base their professional decisions and actions. However, trainees often perform poorly when it comes to correctly evaluating the credibility of online media and critically dealing with online information, which can negatively impact their professional decisions and actions. Competent handling of online sources can only be achieved if trainees systematically learn to search for, critically evaluate and reason using high-quality information. Based on prior research, we define this skillset as 'critical online reasoning' (COR). In this study, a corresponding digital COR training was developed and implemented in the professional education of students in medicine, law and teacher training degrees. In these three domains, competent Internet use and COR skills play an increasingly significant role in professional practice. As prior research indicates, COR-related skills can be improved through targeted training. So far, however, existing tools have only addressed partial COR facets and mostly in a domain-independent manner; no comprehensive training addressing all online information processing stages for these three domains has been available. In this paper, the theory- and evidence-based conceptualization and implementation of the COR web-based training according to the five main phases of the well-established ADDIE (Analysis, Design, Development, Implementation, Evaluation) model of instructional design is presented. The COR training comprises tasks embedded into realistic search problems regarding relevant topics in the respective domains and thus provides an authentic learning environment. The initial evaluation findings indicate that the COR training can be transferred across and improve trainees' COR skills in the three domains. At the same time, some limitations became apparent, which led to calls for further development and the professionalization of training that is presented here.