Switch your voice on and let the sound flow out through your nose.
Hum "mmm" and feel your lips and nose vibrate.
Hold the hum for two seconds, then say: man, swim, come.
✅ Check
Hum "mmm" and pinch your nose shut. The sound should stop instantly. If it keeps going, air is escaping through your mouth and it is not a true /m/.
⚠️ Common trap
Do not let the sound stop short at the ends of words. In "swim" or "come", keep your lips closed and let the hum continue through your nose. Do not pop the lips open into an extra vowel.