Place your upper front teeth gently on your lower lip.
Blow air through the contact so you hear a steady, soft hiss.
Keep your throat silent: /f/ has no buzz at all.
Hold the hiss for two seconds to make sure it is stable, then use it in words.
Now say: fall, safe, phone.
✅ Check
Say "fan" then "van" with a hand on your throat. "fan" is pure air; "van" buzzes. If both buzz, you are voicing your /f/. If neither hisses, check the teeth-on-lip contact.
⚠️ Common trap
Using both lips instead of teeth on lip. Lip-against-lip gives you a /p/-like or breathy sound, not /f/. Check a mirror: you should see your upper teeth touching your lower lip.