The khayshum is the nasal passage from which the sound of ghunnah resonates. It is associated with nun and mim, but it is not a separate consonant called nun or mim.
Nun's principal tongue contact was discussed at point 10; mim's lip contact at point 16. In particular pronunciations of both letters, nasal resonance becomes prominent. The resonating region is the khayshum.
Slowly pronounce أَنْ and أَمْ, attending separately to tongue/lip closure and nasal sound. The length and required forms of ghunnah are determined by their own Tajweed rules.