well, I use Linux. In this I have Bolnagri keyboard which almost works like roman to hindi (phonetic)
example, for - for यात्री, I have to type - yatxrI.
t is for त and x is for ्. (halant as we say)
tx = त्
txr = त्र
i = ि and I = ी ....... (I = Shift + i)
If you want इ, then press Alt and i together. If you want ई, then press Alt, Shift and i together.
कक्षा = kkxSa
kx = क्
S = ष
kxS = क्ष
if I want to type, तथा, then it is tTa.
t = त
T = थ
a = ा
If you want to start off with अ, then press Alt + a. For आ, Alt + Shift + a.
This is easier than transliteration. A proper phonetic keyboard layout.