Once we arrived in Villazon, following our train journey from Oruro, we decided to walk to the frontier as it was apparently only 1km away. The road was easy enough, always straight from the station. The walk gave us the chance to change our leftover Bolivianos into pesos, as we had heard that there were no money changers in La Quiaca, the Argentinian frontier town.
Sure enough as we hit the town of Villazon there were plenty of casas de cambios and we had no problem changing the Bolivianos. All-in-all it probably took us 15 minutes to reach the frontier, located on the bridge. They seem to have recently changed their systems, as now the stamping out and stamping in are done at virtually the same place. No more stamping out of Bolivia and then walking across the bridge to get stamped in on the Argentinian side.
Now you simply cross the bridge and head to the building on the right. Window 1 collects your immigration form from entering Bolivia and stamps you out. Shuffle along and window 2 stamps you into Argentina for 90 days. Easy as that and no entry or exit fees to pay.
From here you can walk to the bus terminal to catch buses into Argentina. Head straight out and then turn right at the end of the road. At the roundabout turn left and then right uphill on the pedestrianized street that will take you to the terminal.
There are buses leaving every hour or so for Salta and other nearby destinations, but if you are going straight to Buenos Aires it may be worth taking the bus from Bolivia instead.