This is super-easy process that anyone can do with with just a screwdriver. This mod covers all BMWs with the N54 3.0 liter twin-turbo engine.
My 2008 BMW 135i was the obliging patient for this demonstration.
Step 1: Locate Air Box Under the Hood
Air box is large square box on the top of the engine.
Step 2: Remove Clips on the Air Box
There are silver clips around the top left, front, and right of the top of the air box. Use a screwdriver to just pop them off of the air box. Don't worry about the clips falling off entirely. They are also attached at the bottom and will just hang to the side until you are ready to pop them back onto the top of the air box.
Step 3: Remove Stock Air Filter
Pry the orange-and-cream stock air filter out of the air box. This is very easy and obvious once you touch the air filter. You aren't going to hurt anything, so just grab the orange edges and pull upwards.
Step 4: Clean the Empty Air Box
On the left side of the top of the open air box, where the ridges are, you will note some debris. Use a vacuum cleaner or a moist rag to clean that stuff out.
Whatever you do while you are cleaning, do NOT let any debris fall into the far-right separated area on the top of the air box--this is your intake and debris falling in there can damage your engine.
Step 5: Drop in the Helene Performance Streamline SE Filter
Doing your best to drop it in vertically, place the air filter into area vacated by the old one. Wiggle it around until it goes in perfectly flat.
The almost-flat side of the filter goes up, and the heavily ridged side goes down.
Make sure the rubber gasket that runs around the open intake hole on the right sits perfectly in the slot that runs around the intake hole.
Step 6: Slide on the Top of the Air Box
Line up the 3 tabs at the top-rear of the air box cover with the slots they go into at the back of the airbox, then slide the air box top toward the back of the air box until the tabs are in their slots and the air box top fits completely over the air box body.
Step 7: Attach the Clips That Secure the Air Box Top
Using brute finger strength, press back into place all of the clips that secure the air box top to the air box.
Step 8: Marvel at the Improved Air Flow
Here is a graph of the decreased resistance to flow seen in a similar filter on a BMW Z4.
I don't have a dyno to tell you if there is any horsepower improvement, but I feel comfortable saying that this is a worthwhile performance and reliability mod for the n54 engine.