Don't over-think it. Just take a ride down and get it done.
Here's what I took with me when I went to get my permits:
Photo copy of passport and driver’s license on one sheet of paper
Vehicle title and registration, and copies of each
Credit card for payment
The first time you ride into Mexico you'll probably be nervous until you get into the country a little ways and get past your first couple of military checkpoints, then you'll be grinning from ear to ear the rest of the trip.