Individuals vary substantially in their language skills. The Individual Differences in Language Skills Test Battery (IDLaS) is a tool to assess variability in (1) linguistic experience, (2) general cognitive skills implicated in language, including nonverbal processing speed, working memory, and nonverbal reasoning, and (3) linguistic processing skills, including word- and sentence-level production and comprehension. The test battery was initially developed for Dutch language users. Building on this work, we recently developed a German version (IDLaS-DE). IDLaS-DE consists of 30 behavioral tests that have been validated in a large group of German speakers, aged between 18 and 30 years. In addition, we have developed a web platform that researchers interested in assessing language and general cognitive skills can use for their research purposes. Here, we provide a guide for creating and running customized studies online via this platform. The IDLaS-DE web platform and all its services are free of charge and accessible at https://www.mpi.nl/idlas-de.