drill a proper-sized hole, tap it, put some kind of sealant on the barb and screw barb in. seal again on the outside and inside with plumbers goop or some other type of sealant (just to make sure; you dont want a leaky res
).
which brings up a Q of my own, if you use brass barbs, do you need to tap the plastic? or could you just screw em in the holes?