There are two ways to do this:
Because DCI TML Ismail is a legacy font, typing a Tamil character requires pressing the English key that corresponds to that specific Tamil sound or shape. The layout does not follow the standard phonetic (transliteration) method or the official Tamil 99 layout. Instead, it uses a unique shape-based and phonetic hybrid mapping. Vowels (Uyir Ezhuthukkal) dci tml ismail tamil font keyboard layout