For the question of typing: The Vedic accents are not available on the standard keyboard layouts, it seems.
Which leaves: (a) direct input individual selection of characters from a character map (e.g., BabelMap); (b) almost-direct input via text editors where the unicode value can be keyed in and pressing Alt-X converts it to the glyph (e.g., LibreOffice Writer; BabelPad); (c) writing custom keyboards (OS dependent); (d) mapping a font in xelatex (here, using a made-up word, which GoogleTrans currently reports as meaning black powder):

Fine detail adjustments in the typesetting might be needed, depending on the actual words used.
(This mapping also implies that direct manipulation via lua code in lualatex should also be possible.)
Experiment shows that Latex macros of the sort \newcommand\myaccenta{\symbol{^^^^0951}} are seen as extraneous boxes at the type-setting stage and interrupt the letter-shaping mechanism.
Also, interesting news: a revision and update of the skt package seems to be in the pipeline. (Typing Sanskrit in TeX)
Code for the mapping example:
\documentclass[12pt]{article}
\usepackage{xcolor}
\usepackage{fontspec}
\setmainfont[Script=Devanagari,Mapping=vedic,Scale=1.5]{Noto Serif Devanagari}
\newfontface\translitd[Mapping=devanagari-to-iastb,Scale=1.1,Colour=red]{Noto Sans}% This is for the IAST-based transliteration only.
\newfontfamily\englishfont{Noto Serif}
\usepackage{polyglossia}
\setdefaultlanguage{hindi}
\setotherlanguages{english}
\begin{document}
\section{\textenglish{Mapping}}
\textenglish{Test text:} कालेमूनि {\translitd कालेमूनि}
\vspace{12pt}
\begin{tabular}{ccccc}
,, & कालेमूनि,, & कालेमू,,नि & काले,,मूनि & का,,लेमूनि \\
\end{tabular}
\vspace{12pt}
\begin{tabular}{ccccc}
.. & कालेमूनि.. & कालेमू..नि & काले..मूनि & का..लेमूनि \\
\end{tabular}
\vspace{12pt}
\begin{tabular}{ccccc}
(( & कालेमूनि(( & कालेमू((नि & काले((मूनि & का((लेमूनि \\
\end{tabular}
\vspace{12pt}
\begin{tabular}{ccccc}
)) & कालेमूनि)) & कालेमू))नि & काले))मूनि & का))लेमूनि \\
\end{tabular}
का,,ले..मू((नि))
\end{document}
The .map file (to be converted to .tec format with teckit_compile):
; TECkit mapping for TeX input conventions <-> Unicode characters
LHSName "vedic" ;
RHSName "UNICODE"
pass(Unicode)
; ligatures from Knuth's original CMR fonts
U+002D U+002D <> U+2013 ; -- -> en dash
U+002D U+002D U+002D <> U+2014 ; --- -> em dash
U+0027 <> U+2019 ; ' -> right single quote
U+0027 U+0027 <> U+201D ; '' -> right double quote
U+0022 > U+201D ; " -> right double quote
U+0060 <> U+2018 ; ` -> left single quote
U+0060 U+0060 <> U+201C ; `` -> left double quote
U+0021 U+0060 <> U+00A1 ; !` -> inverted exclam
U+003F U+0060 <> U+00BF ; ?` -> inverted question
; additions supported in T1 encoding
;;U+002C U+002C <> U+201E ; ,, -> DOUBLE LOW-9 QUOTATION MARK
U+003C U+003C <> U+00AB ; << -> LEFT POINTING GUILLEMET
U+003E U+003E <> U+00BB ; >> -> RIGHT POINTING GUILLEMET
; Here are the Vedic accents
U+002C U+002C <> U+0951; ॑ ,,
U+002E U+002E <> U+0952; ॒ ..
U+0028 U+0028 <> U+0953; ॓ ((
U+0029 U+0029 <> U+0954; ॔ ))
The key to the mapping is to choose combinations of characters that are easy to type and unlikely to be used elsewhere in the text. In this case, double comma ,, double dot .. double left bracket (( and double right bracket )). But other characters can be used, if desired.
As an aside, the font designer designs the font, including the marks. Here's a random sample:

latexwith thedevanagaripackage. The\dnmacro here is redefined for using translitterated text. – egreg May 23 '12 at 16:02