Its pretty simple and straight forward:
1. They entertain you only 15 min before the scheduled time.
2. When you reach the floor, I think 4 th - there are 2 lines to enter the room. One for the walk-ins (long as you guessed). The second takes you straight to the security officer.
3. The officer gets you the token from one of the counters.
4. While you wait for the number to be called - get the FP done in one of the cabins in the room (they will tell you)
5. You can also get your photo done if you don't take the ADIT photos with you. Actually I will suggest you get it done there. It costs $11 for 2 photos there - in the market it will cost you atleast 10 if not more.
6. Once you number is called - give them the approval letter (I suggest you give the courtesy copy as it has your address - and thats where they send the GC. If you give the attorney's copy - the GC may be sent to the attorney - thats what the officer there told me), infopass copy, photos, I94, EAD, AP if any.
Thats it - they will stamp the copy and you are on the way out. If you take an early morning appointment - it should take about 30 min in all. If later in the day - it could be anywhere between 2 hours to 3 hours.
Parking - there is a parking lot across the street from immigration office. They charge about $7 for the entire day.
Security - Ask your wife not to carry nail clippers as she will lose it while being thru the security
Hope this helps.