Seriously, to replace the original levers, you would probably put on both new levers at the same time.
Install the aero lever in the appropriate place.
Install the interrupter lever at the appropriate place.
Cut the cable housing appropriately... one piece goes between the two levers, and one between the interrupter and the brake (or the housing stop on the frame).
When installing the cable into the aero brake lever, slide the cable through the two sections of housing, and the interrupter as if it were a single piece of housing.
Adjust as if the interrupter doesn't exist (the interrupter should be in the non-braking position during adjustment).
If you look at the routing for aero brake cables, an interrupter gets installed into the line by cutting the casing right where it exits the bar wrap.