Set your upper teeth on your lower lip and blow gently, as for /f/.
Now switch on your voice: hand on throat, feel the buzz join the airflow.
Keep the contact light so air keeps hissing through. If the air stops, you have drifted into /b/.
Hold a long buzzing "vvvv" for two seconds, then use it in words.
Now say: van, have, love.
✅ Check
Say "fan" and "van" with a hand on your throat: only "van" buzzes. Then say "vest" and "west" in a mirror: for "vest" your teeth touch your lip; for "west" your lips round instead.
⚠️ Common trap
Replacing /v/ with /b/ (lips pressed together) or /w/ (lips rounded, no teeth contact). Both lose the friction. Your teeth must touch your lower lip and the air must keep flowing.