You should fill coolant before starting the car or running the car for an extended period of time (where the car temperature gage indicates the temperature has risen to normal operating).
If the car has been running and is at normal operating temperature, turn off the car and let the car cool for a few hours.
Open the hood and locate the coolant fill cap near the front of the engine compartment.
Slowly remove the fill cap.
There is a float that should provide indication of your level. An attached indicating plate tells you how to read the float (see pictures below).
If needed, you have a screwdriver in your toolkit in the trunk. Use the phillips end to open the bleed screw near the fill cap. Just a few turns should be necessary to create a path for the air to escape.
Slowly commence filling the system with the appropriate mix and type of coolant*. Do not overfill. Remaining air should leave through the path created by the bleed screw.
*Note: BMW Coolant is recommended as it is appropriately formulated for our aluminum block engines to minimize electrolysis and wear of the aluminum block. BMW coolant is concentrated and requires dilution with distilled water to a 50/50 mix. Distilled water is recommended as it removes any impurities from the water that may promote electrolysis.
Once the float indicates full and coolant emits from the bleed screw, replace the cap and close the bleed screw. Be sure not to overtighten or strip out the bleed screw.
Start the car and crank the heat. Let the car run for about ten minutes. Watch for indication of leakage from the cap or bleed screw. If the low coolant light comes on while the car is running, turn off the car and check the level.
Turn off the car and repeat fill and bleed procedure as described above to verify the system is full.