You can walk up to Av. Luis Maria Campos (maybe 2 blocks from where you live)
There, get on the 29 bus heading North.(there's 2 options for that one: Olivos and Parque Sarmiento= It doesn't matter which)
Ride up to Barrancas de Belgrano. (7 minute ride) and take the Train (TBA) that goes to Tigre. Ask for a "boleto de ida y vuelta" (2 way ticket) to San Isidro. San Isidro is the 6th stop. (one after Acassusso)
There's a "nicer" way to get to San Isidro.
Do the bus thing (60 line or 59 line, this one to La Lucila) but up to Estacion del Tren de la Costa in the Olivos neighbourhood. (25 minute ride from the place you are at). This stop is also called Estacion Mitre en Olivos.
You'll see a bridge "over" Av.Maipu: there you need to get off the bus, get on to the escalators and you'll be in Estacion Olivos del Tren de la Costa. There, get a 2 way ticket to Estacion San Isidro.(15' ride)
when you get off the train,walk forward,cross the train line,that,s Calle Belgrano(the main street)and if you continue along there you,ll pass the cathedral,more or less,and then Tren De La Costa. and it,s lovely there,,,Sat and Sun best for markets,,,