About midway up the passenger side of the engine, at the very back of the engine bay, you will see a 19 mm bolt that is screwed into the side of the block. It's kind of hard to get to, you'll need a universal swivel to obtain the correct angle on your socket. Cover the O2 sensor with a rag. Once the bolt comes out, it pours all over the exhaust manifold and runs all over the place. Just prepare yourself for a mess. It's best to run the car up on ramps or jackstands, it helps when you are filling the system to vent the trapped air out. Just fill it slowly with a 50/50 mix of BMW blue and distilled water, nothing else. This of course after you have re-installed the plug with a new copper crush washer.
Jr