Not a perf expert but basic gist of driftdown is to use the aircraft energy in one of three ways:
to clear enroute terrain - usually achieved by selecting maximum continuous thrust on the good engine, letting the aeroplane speed reduce towards L/D Max (green dot on some types) and then selecting a speed on elevator mode (flch, open descent etc). As you descend into more dense air the good engine is able to deliver more thrust leading to an ever shallowing descent until you eventually reach driftdown altitude.
max range driftdown - applicable to ETOPS. Involves sticking to a particular speed schedule to maximise the range of the aircraft during driftdown to get as close as possible to a suitable enroute diversion
relight envelope - descending within a particular speed range in order to perform a windmill assisted start