I'm grinding my way through FreeCAD.
Battling with the poor readability of the default Sketcher view (some weird font outline, lines too thick...) I stumbled upon a signature of this guy
https://forum.freecad.org/viewtopic.php?p=601399#p601399
A comprehensive lecture on sketcher! Reaaallly sweet, in several languages - English, auf Deutsch, en français, en español.