As you approach the border, you will want to be aware of the lane that you are driving in before you cross. Which lane you will use depends upon what you are bringing into Mexico. Generally there are multiple lanes labeled "Nothing to Declare - Nada que Declarar" and one or two lanes to the far right labeled "Declaration Lane - Carril de Declaración."
Use this lane if you have nothing beyond the personal exemption.
If you are bringing more than US $75 worth of merchandise per person, but not more than US $1000 (amounts over US $1000 require the use of a Mexican customhouse broker), you may declare it by driving into the customs facility thru the declaration lane (carril de declaración). There you will be able to fill out the proper documents (see example below). The duty is usually 25% of the value of the merchandise and may be paid in a nearby bank (sometimes located in the customs facility). Once your duty is paid, you are then required to press a button on a stop light. If you receive a red light, the officials will inspect your merchandise to make sure you paid the correct duty. If you receive a green, you may be on your way.
Even if you have nothing to declare, you still could be randomly selected for secondary inspection. As you are crossing the border, you will encounter a stop light that will either give you a red or green light. A red light means you must pull over into the customs facility for secondary inspection. A green means you may continue on your way although customs officials still have the authority to ask you to pull into secondary. The customs system is set up to register the weight of your vehicle and the size. Generally heavier or larger vehicles will get a red light, but all vehicles can potentially receive a red light as there is an element of randomness to the the system.
If you are selected for secondary inspection, the officials will generally take a look in your trunk and glance inside the vehicle to see if you are bringing anything that should have been declared. The process takes a couple minutes or so unless they find something undeclared. If you do not declare something that you should have, depending upon what the items are, you may have to pay the required duty, risk getting your merchandise confiscated, or go to jail.
Tourist cards are available at border crossings, usually just as you cross. At the El Chaparral border crossing in San Ysidro, California, the Mexican Immigration office is on the right in the SAT building. Read more on how to get a Mexican tourist card at El Chaparral.
Baja has no vehicle permit requirment, however if you do need one, they are issued by Banjercito.
Have a fun trip in Mexico!