I used the cutoff wheel attachment on my Dremel tool to score the flap seam. For this, you will want the big Billy-Bad-Ass cutoff wheels. Not the normal ones, or even the heavy-duty ones, but the ones they keep in the back of the hardware store with the illegal fireworks.

Now that you mention it, a regular old heavy-duty cutoff wheel might work fine. I used the BBA one because it has a much wider diameter.

Notice that the score mark is to the INSIDE of the line where I will score the left-hand side, by about...1/16" of an inch. This is because you will want the flap to fit nicely inside the joint when the side is folded up.

Also notice that the back side is buckling a bit. I'll get to that in a minute.