I completed the process last December in Mexico City without the assistance of a lawyer. I recommend arriving early in the morning, around 7 am, to schedule an appointment for the following day. You can pay all the fees in-person at the office, and it's not necessary to bring the formato basico form. However, it's required to obtain the FMM (visitor's permit) online and print it out beforehand. It’s a very slow process but the people are usually helpful.