it's so ezy you just find that software it called keyman
so install it to your pc
when you typing a letter you open the word doc and go to program and open that software "Keyman" so it come to task bar then you change your fonts it should be sinhala fonts after that right click that keyman icon so then you can see a sinhala letter "a" select the sinhala letter and just type what you need to type that it if youi need that software send me a PM