LibreCAD

Cross-platform Open-source 2D CAD

Wiki, Howtos, Tutorials: https://dokuwiki.librecad.org

Online manual: https://docs.librecad.org/

Source code: https://github.com/LibreCAD

Many questions are already asked meanwhile, so please use the forum search or wiki search and maybe  you possibly find a quick answer for your problem.
Very often asked questions have their own place at our wiki FAQ