Hi Chefgary

Well first off, check your owners manual, or google up the manual for the specific head you have. The head maker's website probaby has that info.

Generally speaking to flush a marine head:
1. Determine if you are pumping overboard or to the holding tank. Find the diverter valve and switch it to the right direction. If overboard, make sure the DISCHARGE seacock is open; if to tank, close seacock.

2. AT the head, your two buttons are probably a water fill button and the actual flush button. So:

1. Is your head raw water or fresh water? If raw water there will be an INTAKE seacock that needs to be opened to fill the bowl; if its freshwater, there will be a valve to the toilete on the line from a freshwater tank. You have to find these and allow water into the bowl.

the second button--probably the rubber-booted one--is your flush button.

Hint: Discharge lines are 1-1/2: diameter; water intake are usually 1" diameter.