Living in McAllen, which is about 15 miles from the Mexican border town of Reynosa, I cross across to the Mexican side several times a year. I go to eat roasted corn on the cob (elote) and enjoy coffee at Cafe de Paris. Their sweet bread (pan dulce) is amazing.
For such crossings, you go through the obligatory scanning of your bags if you are walking across with any. If you are crossing by car, there is a red or green light for each lane that randomly indicates if you should pull over into the inspection lane (when you get the red light). Pop open the trunk, answer their questions, and off you go. Such crossings are permitted without any further formalities within a radius of 20-22 km into Mexico. Some websites say 35 km. Always a good idea to have your US passport with you – you are going to need it to cross back into the USA anyways. Or, if you are a foreign national, have your passport with the multiple entry visa stamp for the USA in it to avoid problems re-entering the USA.
Now, if you plan on traveling beyond the 20-22 km into Mexico, you are required to fill out a form unofficially known as the Mexico Tourist Card and officially known as Forma Migratoria Múltiple (FMM) or Multiple Immigration Form. While you can fill the form out online and print it, without the stamp or seal, it is not valid. You can pay the fee online but, be sure to print the receipt as you will need it to get the permit (also print it) stamped. The Mexican Immigration Services known as the National Institute of Migration (INM; typically their office is as soon as you cross over into Mexico) charge $35 (if you don’t have your receipt for proof of having paid it online) to stamp the form. The permit is valid for 180 days. The multiple in the name of the form refers to the form being multipurpose; air or land and not, necessarily multiple entries as that varies from border regions. Also, the form is purely for tourist visits and not for employment activities. Be careful of providing information to privately run websites that have an official-looking questionnaire and create the form online for you and charge a processing fee. Those sites are no different than sites in the USA that allow you to enter a passport application online. No affiliation with the embassy or the government; just providing an easy-to-use interface and charging a fee for that service. All good, all legal.
The above applies to both, entry by land or air. For air – I have never flown into Mexico from the USA but, read this online – the airlines provide a form before landing that you fill out. Same form as the link below and shown in the picture above, you just hand-fill it.
The good news is that the Mexican government also has a website where you can enter the information and print the form out. However, when I did that on my last trip, the officer at the Reynosa border said that it was invalid because I had not printed it and I showed him the filled-out pdf form on my phone meaning that he could not stamp it. So, I had to hand-fill the form out there, pay the $35 and have them stamp it. I have an upcoming trip to Mexico where I am entering Reynosa by land and then catching a flight from there. So, does that mean I need a land entry form or an air entry form, or both? I will soon find out. My guess is that since I am not arriving in Reynosa by air and arriving by land, and then traveling by air, I will need to fill out the land version of the form.