The nasal "plosives" of the vast majority of the world's languages are voiced. Voiceless nasals exists but they and their symbols are not included below.
During the production of these nasal "occlusives", the soft palate is lowered to a greater or lesser extent, allowing a portion of the airstream to pass through the nasal cavity. Occlusion occurs in the mouth only; the nasal resonance is continuous. Indeed, many linguists rank the nasals among the continuants.
You can hear each sound by clicking on his IPA transcription.
Bilabial nasal. The mouth is configured just as for the corresponding bilabial stop: the lips are pressed tightly together (see figure 3.7 below).
Figure 3.7 : Bilabial nasal
Labiodental nasal. The lower lip is pressed very firmly against the upper teeth. It is difficult to make a perfect occlusion (i.e. an airtight seal) at this location; for the sound to come out properly, a great deal of air pressure must be produced in addition to the tense articulation (see figure 3.8 below). There does exist an oral counterpart to this sound, but it is very rare and the IPA does not even include it in its tables. (And we won't include it either.)
Figure 3.8 : Labio-dental nasal
Dental or alveolar nasal. The mouth is configured just as for the corresponding dental or alveolar stop: The tongue makes contact either with the front teeth, or with the alveolar ridge directly above them.
Retroflex nasal. The mouth is configured just as for the corresponding retroflex stop: the tongue curves up and back so that its tip or its underside makes contact with the roof of the mouth (see figure 3.9 below).
Figure 3.9 : Retroflex nasal
Palatal nasal. The mouth is configured just as for the corresponding palatal stop: the tongue tip is directed down towards the lower teeth, while the tongue body makes contact with the hard palate (see figure 3.10 below). (It is important to distinguish between the true palatal articulation and that of a dental + [j].)
Figure 3.10 : Palatal nasal
Velar nasal. The configuration of the mouth is very close to that of the corresponding velar stop: with the tongue tip resting against the lower teeth, the back of the tongue makes contact with the soft palate (see figure 3.11 below). But as the soft palate is lowered (to allow air to flow through the nasal cavity), the tongue's movement is more important for the nasal than for the oral sound.
Figure 3.11 : Velar nasal
Uvular nasal. The mouth is configured as for the corresponding uvular stop (the tongue tip remains placed against the lower teeth, and the tongue body is raised far enough back to make contact with the soft palate at the uvula), bearing in mind that, with the lowering of the palate, this is easier to accomplish for the nasal sound (see figure 3.12 below).
Figure 3.12 : Uvular nasal