First, make sure that your heater controls are set in the on (hot position). Next undo the lower radiator hose and drain the system into a bowl or other suitable vessel. re-connect the lower hose and fill the system via the overflow bottle. I half fill and then start the engine with the overflow bottle cap off. As the engine warms I add the remainder of the coolant replace the cap and then let it all get up to temperature. Switch off allow to cool down then top up as needed.
Make sure you use distilled water and a good quality anti-freeze. I use a 50/50 mix of distilled water and Bluecol. Hope this helps.