Lots of different ways you can plumb this. Assuming you flush with sea water and your head is below the waterline (almost inevitable), you need a vented loop in the water supply line to keep siphoning from sinking the boat. The (normally 1 1/2 inch) discharge line may or may not need a vented loop. If you are just going into the holding tank - no. If you have a Y-Valve or similar arrangement that allows you to pump overboard to a thru-hull - you''ll want a vented loop in that line as well. Some vented loops can be fitted with a small tube that attaches to the vent and, from there to thruhull (above waterline) vent (similar to a gas tank vent). Others do not. The holding tank, however, needs to be vented, preferably w/ 3/4 inch tubing. Get good quality odorproof hoses. Look at Casey''s book. Also, I believe the West Marine catalog has a couple of simple diagrams that may be helpful.