Pronouncing nasal vowels can be a little tricky for Americans since they don't
really have any analogous sounds in any English words. However, we do
occasionally use nasal vowel sounds. Most commonly, this can be seen in the
colloquially response to something not understood:
Speaker 1: [mumble, mumble, mumble]
Speaker 2: "Huh?"
The sound of the "u" in "huh" is usually nasalized by American speakers of
English and corresponds to the French /u~/ sound. Variations on this
colloquial sound: "Hanh?" "Heh?" resemble the French /ã/ and /i~/ sounds.
The diphthong /wi~/ sounds a bit like the nasalized proverbial whining
sound waa (which is not the same sound as the wa of "watt").
Others have suggested that the French nasals resemble English vowels
followed by n or ng. This is somewhat true for /ã/ which
sounds a bit like the ong in "long" (though not among speakers who say
"lawng"); /õ/ which sounds like the o of "loan"; and /u~/ which
sounds like the o in "among" or "tongue".
The /i~/ sound is similar to the a of "can't", but not among speakers
who extend the vowel out to "caint" or who pronounce it to rhyme with "font".
Obviously, these sounds are best modelled by a native speaker.