We went to the papeleria in San Antonio Tlayacapan (just a few doors west of the south end of the plaza). It costs a fraction of processing through a lawyer. You’ll need to bring your passport and the return INM form that you normally have to hand in at the airport when you return home. You’ll also need proof of address (utility bills). The owner there will give you a form to fill out showing your name, address, weight, height, etc., and she’ll ask you to return at a later time.
When you return she’ll have all the forms filled out for you. You then have to head to a bank and give them the payment form and pay your fees (4,500 pesos each at time of writing). The bank will give you a receipt. You have to then obtain two copies of the receipt (back and front). Now head to the INM office! I have a full write-up of the process which we experienced in July/Aug of this year, if anyone is interested. Joyce