This is an online service to be found at: http://www.thlib.org/reference/transliteration/wyconverter.php
It is the THL endorsed way to generate easy to scholarly romanized transliterations of Tibetan that preserve the original spelling. It uses THL's own "THL Extended Wylie Transcription" system.
Paste your Tibetan Unicode or Wylie romanized text into the box, specify if you want Unicode to Wylie or Wylie to Unicode, and click "convert".
Prep your text by reading Wylie to Tibetan Machine Unicode to make sure you are precisely following the "THL Extended Wylie Transcription" system in your romanized text.
It is that simple.
It was created by Roger Espel Llima and originally published at http://www.digitaltibetan.org/cgi-bin/phonetics.pl.