Kilometer 21 (Km 21) on Federal Highway 15 is the Aduana: Mexican immigration and customs. This is where you get your tourist visa and your vehicle permit, about 21 km south of Nogales.
It can be confusing, because the offices sit just south of the KM 256 sign. Make a right turn into the driveway. Walk to the Tourist Visa office and get your visa. Ten feet away, inside, is the copy kiosk where you can get the copies you need. Then continue to Banjercito to get your vehicle permit.
Get your papers and start your amazing road trip here. Have you been through KM 21? Any tips for first-timers? Tell us in the comments.