Occasionally, modelers ask about how to make the notches in the formers. There a different methods. The one I like best is with my homemade notchers.
It's difficult to see, but there is a strip of sandpaper glued to the edge of each balsa strip. The wide piece that makes the "T" is a backstop.
Here is the notcher in action. About 10 strokes, back and forth. The backstop keeps it from going too deep.
Sorry, kind of bad photo but, it made a neat notch in this piece of paper.