Study it carefully for there will be a quiz afterward.
JUST KIDDING! The diagram gives you a picture of where the holes will go.
We work on a cutting board, so we don't mark the kitchen table. I find a push pin a good tool for this work, but you may try a needle, tack, or something with a sharp point.
I push the pin through the flap all the way to the cutting board. Check out the last picture in this post to see the holes on the flip side.
The mid point of each edge is the eighth point, which falls exactly in the middle of 15 points.
Again, we mark points in pencil, lightly, spaced about ¹⁄₈″ apart. We make 7 points down the middle and then prick them with a push pin.
Then, we rotate the ruler to make the points on the other side. Hopefully, the diagram at the beginning of the post will make it look clear.