The Office in Mexico City you need to go to is:
The easiest was to get there probably is by metro, which is how I got there. Get off the metro at the Tasqueña stop which is at the end of the Blue Line or Line 2. Walk to the train which is up the stairs and to the right as you get off the metro, get off the train at the Textitlan stop. The train stops in the middle of a highway with a overhead bridge. On one side of the highway are businesses; the other side has government offices and university buildings. Head towards the government offices, as you get off the bridge head left. About one block in front of you will be a group a soldiers. This is the place.
Metro Map. Maps of the Metro are almost impossible to find, you might want to print out this map if you do not have a map.
The officials will not need the originals of your documents but will need a copy, best to bring extra copies of everything just in case.
As of July 2015, there were no costs, its free.
The first time I got it; I was in and out of the office in fifteen minutes, the second time there was a huge line over a block long (in a shady area). But the line moved quickly and surprisingly took less than 90 minutes to get everything done. As I said before, this is easy to do, relax.
As you get towards the front of the line, if there is one, you will be given a paper to fill out, asking for your name etc., fill it out with your black ink pen.
If this is the first time filling out a form in Spanish asking for your name, this might be tricky. In Mexico and in most of Latin American, people have two last names. The first last name or apellido paterno is from their father and the second last name or apellido materno is from their mother. Your first name(s) is referred to as nombre(s). If you do not have a apellido materno, leave the space blank.
As you get to the entrance, office workers will sort your paper work and put it in order. At the entrance soldiers/security will check you for bomb making material justlike at the airport. Water and sodas are not allowed into the building. You will not be allowed to use electronic devices in the building. Your paper work will be collected and you get to go in. Take a seat, move to the left as the seat to the left is vacated. No talking is permitted. There is a bathroom there, this is your chance to use it, it will not effect your place in the line.
When your name is called (probably within ten minutes), you will go to the front counter and sign a paper and you will get your Constancia Federal. Make sure your name is spelled correct. YOUR DONE!!!!
The entrance and inside the building are accessible to wheelchairs. I have no idea about the bathrooms. If there is a huge line around the building, that area is not accessible to wheelchairs. I suspect if you were to go to the front, the officials would sort your paper work and move you to the front of the line.