Well, the letter k [ka] as an example:
kri [the Sanskrit vocalized r]
Ligatures at the end of the alphabet:
Initially, you need a diagram of the Devanagari alphabet to find your was through the dictionary.
Nasals come first of all, grouped under the resp. vowel, not separately (the letter 'a' starts with nasalized 'an'), then the plain vowels including 'ai' and 'au' (a, aa, i, ii, u, uu, ri, e, ai, o, au) then the consonants:
ka kha ga gha
cha chha ja jha
Ta Tha Da Dha Na
ta tha da dha na
pa pha [fa] ba bha ma
ya ra la va sha sha sa ha
Diphthongs made up of two vowel signs come before the consonnants, e.g. 'kaaii' comes before 'kaak'. Letters with a dot below are just variants and are not grouped seperately.