I'm assuming you got the Blue line or a bus into Long Beach; the transit center there has a number of buses (LBT 81, 91-94, 171, and Passport D [magenta line]) that take you to the VA Hospital for 90 cents. From there, across the street from the hospital, you catch the OCTA #1 bus for $1.25 to San Clemente. There you meet up with the North County 395.
The North County bus costs $3.50 for a day pass; you used to be able to pay $1.75 one-way fare and get a transfer, but no more! They got greedy. At the end of the run, in Oceanside, switch to the 101 which takes you to UTC.
UTC is a huge shopping center near UCSD. Chill out there for a while if you wish, then take the 41 to Fashion Valley. There are other routes, but that is the most direct. It costs $2.25, and make sure to ask for a transfer when you get on, as you're paying the fare. With that transfer you can take the trolley from Fashion Valley, all the way to San Ysidro if you wish, which is mere steps from the Mexican border.
Total cost of this leg of the trip, including the $3 LA day pass which you may have already had from the previous leg: $10.90. If you did already have the day pass, the incremental cost is only $7.90 from LA to SD! Compare that to $15 that Greyhound charges, and you might feel this tip was worth a $1.00 donation paypaled to firstname.lastname@example.org.