I. Do. Not. Like. Daggers with rapier, and now they are invading the classes too. Seriously.
Here we see a parry with the sword in quarta and then a strike in quarta to the face or a riverso to the arm.
You are in terza, extended, with the dagger at your wrist. Adversary is in whichever guard such that stringering on the outside makes sense.
You'll close in terza, then, high or low depending on his guard, but without moving the dagger from the wrist.
When he disengages to attack, you parry with your sword in quarta(1), supporting that parry with the dagger as shown, and then you can strike him as described.
(1) That's what the text says, but I am not sure the picture shows really shows a quarta. Maybe once you got the dagger there to support the parry you can turn the hand, and what's shown as the hand position is the end of the riverso rather than the described attack in quarta.