You'll find the MexiCoach bus station on Revolucion near 7th. It is on the west side of Revolucion in the back of a tourist arcade. There is a sign on the roof of the building, but it's easy to miss.
The last time I took the bus it cost $2.50. The bus runs about every 20 minutes from about 7am to 9pm.
Sometimes it is a good deal, other times not. It depends on how many buses and what type of buses are in line ahead of you. Buses headed north (to LA, etc.) take a long time as luggage must be off loaded, inspected, then reloaded.
If you're in the pedestrian line many times you can knock on the door of the bus and pay the driver a fee to board. (Do that before it crosses into the US.)
If you re-board the bus it will take you to the Boarder Station Parking lot on Camino de la Plaza. You can board the bus in the US (at the Border Station Parking Lot) and take it to Revolucion.