First you pass the CO2 tubing through the nut. Then you remove the O-ring off of the male end of the bubble counter and put onto the CO2 tubing. Then you put the CO2 tubing on the male end of the BC and you can slide the O-ring down and screw on the nut. This should make a tight seal. If you find that you are not getting consistent CO2 rates (the problem I was having here) then you can just remove that O-ring and put it where the needle valve nipple is.