For doing the thing @codewiz describes: https://www.youtube.com/watch?v=iy9amRGcCCA
Once you're somewhat familiar with "Part Design" and Sketcher, I recommend getting familiar with the "Part"-workbench and boolean operations (can't remember which tutorials I found useful for this atm).
If the goal is to 3D-print your model, you also need the tool "convert shape to mesh" from the mesh workbench.
@metnix @patrick This was a good one to learn about object construction: draw a 2D sketch, add constraints, then extrude:
https://www.youtube.com/watch?v=uh5aN_Di8J0
@metnix @patrick This more recent tutorial specifically covers the current version of #FreeCAD, but I'm still not done watching it:
https://www.youtube.com/watch?v=E14m5hf6Pvo