Welcome to the forum.
Hope you didn't wait 'till you're holding it, making the inquiry much more urgent.
The marine sanitation system instructions depend upon what is installed on the boat. There are several slightly different systems, with slightly different operation instructions.
The best option is to download the specific instruction and parts manuals for the system on your boat. The toilet, vacuum generator and holding tank all have model number labels.
As an overview, most are Vacuflush systems. To operate, turn on the head breaker and the fresh water breaker. Allow the system to build full vacuum, DC current shown on panel meter stops surging. Add waste to bowl. Lift foot pedal if additional water is required. Step on foot pedal to flush. When using mine with solid waste, I hold the pedal down until the solid has worked through the entire system, as indicated by the audible sound change of the empty plumbing.
Some have a macerator for overboard emptying of the holding tank. Ensure that the boat is far enough offshore before discharging the holding tank. In the engine room, move the seacock from closed to open. At the DC panel turn on the discharge switch. Allow to run until the holding tank is empty.
Note that there are level switches in the holding tank. Those frequently get stuck. On mine, I empty the holding tank, remove the switch assembly for cleaning. I also clean the sidewalls of the holding tank at that time to make it easier to do a visual level inspection.