super simple

click the shifter until it's fully released (e.g. it would be on the smallest cog)
on the Rear Der, turn the barrel adjuster till it's all the way in, then out two clicks.
route the cable through. pull it tight and screw it down in the rear der (5nm)
turn the barrel adjuster out a few clicks.
turn the pedals, and shift.
turn the barrel adjuster out (increasing tension) until the chain is quiet and shifts smoothly through the gears.