I have the Airwolf air/oil separator, so I can't help you with the BDS installation, but I think that the breather hose installation should be about the same. It took me awhile to properly get the breather hose positioned from my air/oil separator to avoid (a) odors in the cockpit when I shut the cowl flaps, and (b) eliminating any siphoning effect which will cause oil to magically appear on the belly of the airplane.
If you have a Garwin, Pesco, or Airwolf vacuum pump, then you have a "wet pump". Basically, this means that the pump uses a little oil from the engine to lubricate the vacuum pump vanes and then expels that oil mist along with the air vacuumed from the instruments into the air/oil separator, along with the crankcase breather air (and moisture). The air/oil separator then is supposed to separate the air vapors from the oil mist inside the cannister and allow the air vapors and moisture to exit the cannister through the breather tube, and allows the oil condensate to be returned to the engine via the return hose using both gravity and a little pressurization from the cannister. If you didn't have the air/oil separator doing this job for you, you would end up expelling a lot of your engine oil onto the belly of your plan.
I've attached a picture of where my breather hose from the air/oil separator exits the left cowl flap. There are also references to this problem in Colvin's book, and the 2009 ABS Reference Library CD.