Richard! ..You undo the “radiator cap” on top of the fresh water coolant tank, right at the very front of the engine and just on top of it, and refill with a mixture of fresh water and anti-freeze.
Pour in the coolant (water + anti-freeze) till it appears to be full, then run the engine, so that the coolant gets pumped around the system and the level in the tank drops, and while running it, keep topping-up the tank till it’s full. You can “dip” that header tank by just putting your finger into it – while it’s cool – and checking that the coolant comes up to the level of the vertical metal rod which stands inside the header tank, about half-an-inch below the open filler ..or just use a pencil to dip the tank, and make sure that the watery coolant is up to about half-an-inch below the open filler neck.
I can’t remember if the coolant hose to the gearbox-oil-cooler is LOWER than the engine’s own drain cock, or if the engine drain cock is lower, but to draw the whole system – INCLUDING both the engine and gearbox oil coolers – you MAY need to disconnect their rubber coolant hoses, too. Can’t hurt.
Incidentally, I just – last year and this – replaced my twenty-five-plus-year-old gearbox oil and engine oil coolers, as the gearbox oil cooler had spring an internal leak, and was bleeding water into its oil ..and thus into the gearbox! I replaced the gearbox oil cooler last year, and as the engine oil cooler was of the same vintage I replaced that one this year ..just to be safe!
It’s also worth REMOVING the ENGINE HEAT EXCHANGER and cleaning out the pipes inside it with a very thin long-handled artist’s paint brush, to remove all the gunk which gathers inside the coolant tubes. My engine (..1989 NC33 Mark II Lehman..) had been overheating, and removing and cleaning the engine heat exchanger has dropped its running temp down from 90 degrees (ouch!) to 70 degrees. The heat exchanger was cluttered with multiple old scraps of zinc anode which had collapsed into the heat exchanger for-ard section. Well worth removing and thoroughly cleaning – but it’s VERY TRICKY to replace onto the side of the engine, as the metal bands which hold it onto the engine are a very tight, very snug fit!