Here is a written description, for those who don't like to watch videos:
Open the footprint editor,
make a new footprint in a project-specific library or another library.
Call your footprint something like VIA-0.6mm.
Make the name and reference designator invisible.
Put in a circular pad, 0.3 hole size, 0.6mm overall size.
Specify all copper layers, un-check any other layers.
Change the pad connection to Solid in the Local Clearance and Settings tab.
Save the footprint into the library you chose.
Close the footprint editor.
Go back to Pcbnew.
Use the "Add footprints" button to drop in the via footprint wherever you want it on the board.
Select the Pad, not the entire footprint, and edit the properties.
In the Net Name: box, type GND or whatever your net is called.
Back at the PCB, Fill or Refill All Zones ("B" hotkey).
Now it becomes a stitching via.
Mouseover and Ctrl-D to duplicate it, or right-click and Create Array to make an array of them.
Adjust the parameters in the Create Array box as necessary to put your stitching vias where you want them.