Its not too hard on an early VN with a surge tank. With a cold engine take the bottom hose off the radiator and let it drain out. Next take out your surge tanka nd empty it out, refit it making sure the small metal pipes from the inlet manifold are not blocked with crud. if they are unblock them.
Next you can remove a heater hose from the engine and blow the coolant out of the heater core. Refit all your hoses and fil the engine with clean water. Take it for a bit of a drive and wnen it is cool drain it via the bottom home on the radiator and refill it. Do this untilt he water is clean. You can add a radiator flushing compound with the first lot of water change if you like.
When you have changed the water a few timesand it's nice and clean drain it again, making sure you get at least 5 litres out, you may need to empty the heater core again as above.
Go and get 5 litres of Nulon Long Life Coolant and then add that to the emply cooling system without diluting it. Next fill the system with clean water and take it for a drive. The cooling system on these cars will pretty much self bleed via the small metal pipes off the inletmanifold that go into the surge tank.
Also check your hoses and if any are swollen or soft etc replace them first. Be careful if you take hoses off the heater tap not to over tighten as you will crack the heater tap.