Nasal sounds use the nasal cavities in addition to the mouth and/or throat as a resonating cavity. If you sense into those parts of your skull while saying those sounds, you will realise this yourself.
The Nasal Consonants are part of the 5 x 5 Matrix of the Consonants of Devanagari:
ङ – n*a (guttural Nasal – produced in the throat); don’t confuse the letter with retroflex non-aspirated D ड
ञ – n’a (palatal Nasal – produced between the roof of the mouth and the body of the tongue)
ण – Na (retroflex Nasal – produced between the tip of the tongue and the top of mouth); make sure to keep a space between the arch and the straight line, otherwise this letter would be confused with P प
न – na (dental Nasal – produced between the tip of the tongue and the top of the upper front teeth)
म – ma (labial Nasal – produced between the lips)
The nasalization of Vowels is signified by these two signs
The first sign ं is used when the vowel extends above the horizontal line of a word.
For example: औं एैं िं र्इं
The second sign ँ is used when the vowel stays below (or on) the horizontal line of a word.
For example: अँ उँ नँ (note that in the last case not the consonant न् is nasalized but the inherent अ !)
Also be aware that there is a slight difference in pronunciation between a nasalized vowel and the combination of the non-nasalized vowel plus a nasal consonant. But this difference is even not always made by Nepali themselves. Especially when you look up such a word in the dictionary, always make sure to look for it in both places.