You will catch the 992 city bus right outside the airport terminal.
Ask for a day pass ($5) and take the bus to American Plaza.
At American Plaza you will board the trolley - Blue Line heading for San Ysidro.
There is a Starbucks in the Plaza, and this is the same side of the station to catch the trolley.
San Ysidro is the last stop and pretty much everyone is heading across the border.
You will follow the crowd up and over the US Customs and down into Mexico.
There are two turnstiles to pass through. If we are meeting you there we will be at the second turnstile.
If I'm late, wait here, I will show up.
There is a third turnstile but you have to make a right turn to get to it.
The one we'll be at is straight ahead of the first one.

See the below route images info for both, but it's real easy.
For the 992 bus, get on the bus at F or G and off at C

I think both the bus and the trolley run every 15 minutes.

If you are catching the bus to Ensenada:

Go through the first turnstyle, then when you approach the second, make a right turn and go through that one.
Cross the street and turn left.
It's only a block or so and you will come to the bus station on a corner.
The bus costs $140 pesos or around $12.50US and it is a nice bus with a movie (in Spanish of course)