First of all it is very important to use BMW coolant that is usually bluish. Other types of coolant have too many minerals, the wrong type of additives and will corrode the internals of the engine.
While the car is cool, add the BMW coolant in to the radiator reserve tank while the engine is idling. ( this is where the cap screws on ) As it gets filled, open up the little bleeder on the thermostat housing on top of the engine, and you should see some bubbles start to release as the engine heats up. Then when the engine is at normal temp i.e. middle of the gauge, then you should see the air bubbles stop coming out and there will be just be coolant, at this time you can close the bleeder. Then drive the car for a 15-20 minutes. Bring the car back and let it cool down some and then while it is idling re-bleed the engine. You will need an area where the coolant can spill into a pan. I usually do this on a lift in a shop because the ground clearance on BMW cars is very low. Good Luck. !