Let your tongue tip rest down behind your bottom teeth.
Raise the back of your tongue until it touches the soft palate.
Switch your voice on and hum through your nose.
Hold the hum, end cleanly with no "g", then say: sing, long, song.
✅ Check
Say "sin" then "sing". Your tongue tip should touch the ridge for "sin" but stay down for "sing". Then pinch your nose during "ng": the sound should stop completely.
⚠️ Common trap
Do not add a hard "g" after it. "Sing" ends on the nasal hum itself, not "sing-guh". Release the sound through your nose and stop. Also keep the tongue tip down, or it becomes /n/.