Actually very easy. Step by step.
1. The formal way is close the air valve and turn on the pump.
2. Using the gatevalve, adjust until you see the water level at the mid-height of
the reactor chamber(excluding the reaction sump).
3. Then slowly turn on the air-valve until you get fine bubbles. Too much air, the bubble will be bigger while too little air, bubbles will be fine but density is low. Adjust until you feel comfortable with the bubble size and density...
4. Let the skimmer run for 5-10mins to stablise, then start adjusting the gatevalve water the water level in the skimmer reactor raise to almost the "neck" portion. Higher water level will get wetter foam while lower water level will get dryer foam.
voila! But seems like my bubble density a bit weak so may need to open the air valve a bit more