To get to Longtan from Taipei Main Station, follow these steps:
Go to the Taipei Main Bus station.
Find a TV screen that shows departures. Look for one that either is directly to Longtan or passes through the community - this will be in both English and Chinese. The main operator will be King Bus.
Go to the appropriate company's window and say "Longtan" - they'll understand. A one way fare should be NT$82 with King Bus.
Ascend to the second floor waiting area and find your company's booth. Look for the "Long Tan" departure gate.
Wait in line. The buses run frequently every 15 minutes during peak periods, but are often quite late sometimes 20 minutes late.
Seating is first come, first served, usually with no assigned seats.
The bus trip from Taipei to Longtan is almost exactly an hour. Once you pass the toll plaza included in your ticket, the bus won't stop, it will be another 10 to 20 minutes.
The bus will pull over in front of the company's ticketing office in Longtan along the main drag. Disembark. King Bus' stop is across the street from the main terminal.
On King Bus there are approximately 3 stops prior to Longtan, but if unsure, your fellow bus riders can help.