Devanagari in Latex

I downloaded RomDev.map from https://github.com/somadeva/RomDev

Then I ran

teckit_compile -u RomDev.map

which produced the file RomDev.tec. In the same working directory, I copied the file from the blog page you're referring to and compiled it, getting, for the relevant part

Then I proceeded to make the files available to the TeX system. With superuser privileges I did (bash shell)

mkdir -p $(kpsewhich -var-value TEXMFLOCAL)/source/xelatex/fontmapping/somadeva
mkdir -p $(kpsewhich -var-value TEXMFLOCAL)/fonts/misc/xetex/fontmapping/somadeva
mv RomDir.map $(kpsewhich -var-value TEXMFLOCAL)/source/xelatex/fontmapping/somadeva
mv RomDir.tec $(kpsewhich -var-value TEXMFLOCAL)/fonts/misc/xetex/fontmapping/somadeva
mktexlsr

The last command is crucial and should not be forgotten. I tested again by running XeLaTeX and the output is the same as before.

Consult Create a local texmf tree in MiKTeX for doing the same within MiKTeX.