Use the version 1 or version 2 documentation to get started, along with the recording of Mike Goodwin giving a lightning demo during the OWASP Open Security Summit in June 2020. An introduction to Threat Dragon is provided by the OWASP Spotlight series, and the Threat Modeling Gamification seminar by Vlad Styran shows how using Threat Dragon can make threat modeling fun. There are a couple of OWASP community pages that give overviews on Threat Modeling and how to get started: Threat Modeling and Threat Modeling Process. The easiest way to get in contact with the Threat Dragon community is via the OWASP Slack #project-threat-dragon project channel, you may need to subscribe first.