The best way I have found is after installing the box, hoses, pump, etc. but before hooking up the drag link, fill it with fluid and simply turn the steering wheel stop to stop ten or so times without the engine running.
This helps prevent the near foaming that can happen when air escaping the box gets whipped up in the pump and sent back to the box.
That "low fluid" growl and that milky look when the reservour is actually full is caused by air. Let it settle out naturally. There is no need to replace the fluid.