There are fresh water and sea water wash-downs. And sometimes the system can be used both ways. So you need to sort how the water source enters the pressure pump. There will be a valve that you turn to open, if it's a dual water source system, you'll have a valve to select fresh or sea water.
Then you have the power to the pressure pump that you need to turn on. Most that I've worked with have a pressure sensor so when the water pressure in the line reaches a set point the pump turns off.
Then there is usually a valve close to the hose connection that needs to be opened but not always.
Some of these valves can be kind of hidden. Follow your hose back to the pump and then back to the water source connection.
Summary: Open valve to water source, turn on pump, open valve to hose. Wash down what you want to clean.
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