FreeCAD

tutorials

• https://www.youtube.com/watch?v=_tyuIIk0M3E

Sheet metal plugin

• https://www.freecadweb.org/wiki/Sandbox:Sheet_Metal

3D navigation

• Set '3D navigation' to 'touchpad' in Preferences

Draw a rectangle

• https://www.youtube.com/watch?v=AxqtJ-306HE

Extrude

1. use the 'Draft' workbench and 'Trim or Extend' to extrude a face
2. or in 'Part' workbench use 'Extrude'
3. ... more ways to extrude: https://www.youtube.com/watch?v=jlmd5_lnwXM