Continue scoring along the lines made (very carefully as per the first step), and gradually apply more pressure as the cuts become deeper.

Reduce pressure as you sense that you are getting close to cutting through the plastic.

As you just break through the plastic on the flat side (not on either of the round sides), you will feel the blade start to scrape on metal. At this point, start to gently slice around the cut / score line all the way around the casing, avoiding cutting into the metal as much as possible.