Step 4: Step 3: Punch the Cardboard
Just punch a hole in the cardboard. Make sure the hole is aligned in the center of the angled slits, but doesn't hit them.
The hole shouldn't go below where the straight edge begins -- that will be covered by thread eventually.
I don't have a hand hole-punch, so I used the binder punch I found in the apartment when I moved in. That's harder to align. I messed up one already before I got these pictures done. Ha.
But if you're lucky and have a hand-held hole-punch, this part should be pretty easy for you.