Now go forth, trainer. Catch that Haunter on Route 214, level it to 37, and watch the evolution you’ve been denied for years. Welcome to the complete Sinnoh experience.
(often referred to as the standard for "NTEVO" or No Trade Evolution patches). Open Your ROM : Load your file into the application. Select "Change Impossible Evolutions" : Check this box.
If it attempts to evolve and stops, or if nothing happens, you likely applied the wrong patch version (e.g., European patch on a US ROM). Try another patch.