Put your tongue tip between your front teeth, exactly as for the voiceless "th".
Switch on your voice: hand on throat, feel the buzz while air flows over the tongue.
Do not press hard or stop the air; it should feel like a gentle vibrating leak.
Hold a buzzing "thhh" for two seconds, then move into words.
Now say: this, mother, breathe.
✅ Check
Say "they" then "day". In "they" your tongue is visible between your teeth and air keeps flowing; in "day" the tongue taps behind the teeth and the air stops. Hand on throat: both buzz, so the mirror is your judge here.
⚠️ Common trap
Swapping this sound for /d/ or /z/, so "they" becomes "day" and "breathe" becomes "breeze". Keep the tongue between the teeth and keep the air moving; a /d/ blocks the air completely.
Both th sounds side by side, with the language-by-language mistakes: the th sound guide.