The coolant requirements for you engine can be found in the operating manual for the bike from which the engine orginated. Follow the mixing instruction from the coolant manufacturer or mix slightly more coolant and water (50/50) than your cooling system holds and with the engine cold, fill the radiator/header tank. Keep an eye on the level for a minute and replenish as the coolant is drawn into the system.
Once the level stabilizes, start the engine, but leave the radiator cap off and run the engine until the fan switches on three times, then switch off.
Let the engine cool with the cap off, then top up the radiator/header tank to the “max” mark. Put the cap back on, and re-start the engine.
Run the engine again until the fan switches on three times, then switch off and let the engine cool down.
The coolant should have returned to the max level, but if it’s below the mark, then top up to the mark again and check for leaks.
However, if the coolant level is above the max mark, then there’s still air in the system, which means the procedure should be repeated.