Walk from Erronda Kalea to Bilbao Abando. Plan on about 10 minutes. Take the rail option to Donostia / San Sebastián. Total ride is about 1.5 hours.
From San Martín 53, head to the Euskotren stop for the Hendaye run. Ride is about 35 minutes. Taxi fallback is about €30 if you do not want to deal with bags.