To commute from San Telmo to San Isidro: take the 126 or 28 buses to Retiro. Ride should take approximately 25/30 minutes. Go to platforms 1 & 2 of the "Bartolomé Mitre" line (note that there are THREE train terminals in Retiro, you want the one on the far left, Av. Ramos Mejía and Libertador). Return ticket will cost you 1,40 pesos. Just to make sure you are on the right train, you should pass the following station: Lisandro de la Torre, Belgrano, Nuñez, Rivadavia, Vicente López, Olivos, La Lucial, Martínez, Acassuso, arriving in San Isidro after about 35 minutes. Your train should be white & light blue, with air conditioning. If you see a yellow & red run down convoy, DO NOT use it. That one's meant for the cartoneros.
I hope this helps! Enjoy the ride. Send me a private message should you need anything else.