Take a flight from Jakarta, Makassar or Bali to Jayapura, capital of the province of Irian Jaya/Papua, then, a connecting flight to Wamena will take you to the heart of Baliem Valley within the very same day.
A number of travel agencies regularly organize tours to the Baliem valley. (Click on Find Travel Agents)
For your own safety, when intending to trek alone into the interior, you are advised