San Diego has trolley service down to the San Ysidro border (fare $2.50) via the Blue Line. From there you walk across into Tijuana. Where The Road Forks+1 After crossing, you can take a cab to the Tijuana bus station.
Buses operate to Mexicali, with connections onward to San Felipe through Autobuses ABC. Rome2Rio+1
Call ahead to confirm schedules and fares. In Mexico, ABC offers a toll-free line: (01 800) 025-0222. You can also check their schedules and book tickets on their website. ABC+1

The Tijuana Bus station is at:
Central Camionera de Tijuana
Blvd. Lázaro Cárdenas & Río Alamar S/N, La Mesa, Fracc. Murúa
Phone: (664) 621-2982
Open 24 hours, every day.
If you prefer to travel via Calexico and cross the border into Mexicali, the Greyhound station lies just across the border in Mexicali. Greyhound runs several daily trips from San Diego to Calexico (about 4 hours, ~$20 USD one way) and from Los Angeles to Calexico (~6 hours, ~$25) via their web fares. Rome2Rio From the border into Mexicali, take a taxi to the bus station located about 3 miles south of the border:
Calz. Independencia 1244, Centro Cívico, between Calz. A. Lopez Mateos and Calz. Anáhuac
Phone: (+52) 686 556 1903

The San Felipe Bus Station stands on Airport Road (Mar Caribe South), near the 7/11, Pemex station, and the Sebastiani sculpture. It opens early (about 5:30 a.m.) and closes after the last bus from Mexicali (around 10:30 p.m.). No phone service is available for the station.
Between San Felipe and Mexicali, ABC now runs direct service about five times daily. The trip takes about 3 hours and costs roughly between 360 to 700 MXN. Rome2Rio+2Rome2Rio+2

There is also a bus route from Tijuana to San Felipe via Mexicali on ABC. Buses between San Felipe and Ensenada have resumed, initially limited to Fridays, Saturdays, and Sundays. Reservamos+1