You go straight out to National's area in the garage, you don't stop at the counter. The rows of cars are labeled by signs. You go to the row labeled "Emerald Aisle". Pick any car in that row, stop at the booth on the way out to process your paperwork and you are on your way!
I just noticed you said Sanford, I'm not sure if they have an Emerald Aisle there. Check with a lot attendent.