Start the car and push the sway bar all the way up by hand. This will put the level controller into the emptying position. Hydraulic fluid will slowly flow into your catch pan. Add new hydraulic fluid as the reservoir empties. If you first pump out the old dirty oil from the reservoir, you need about 2 litres to flush/fill the system. The reservoir itself holds about 1 litre.
Once satisfied, switch off the car. Reconnect the sway bar. Top up reservoir to the mark and reconnect return line.
Look how clean the oil looks. You can barely see it in this photo. I purchased the oil from NAPA when it was on sale for $9/litre. The part number is; AIC 0009899103. Its the standard Febi branded ZH-M fluid.