Have just realised that in my tutorial I didn’t explain how to cut “standard” spar slots. I never use them on the sort of wings I make, scale fabric covered, but of course this method is just as useful for sports models!
When the blanks have been sanded to shape and are still held between the templates, make the vertical cuts (shown in red) with either a razor saw or junior hacksaw.
With the ribs still held between the templates the balsa “tabs” will now break off quite easily; then finish the slots with a small file to get neat corners before removing the templates.