A lip trill is one semi-occluded vocal tract exercise: air passes through relaxed lips while the voice sounds behind them. Singers and clinicians use several exercises in this family, including humming and straw phonation; no single one is universal or automatically correct for every voice.
Begin without pitch. Let the lips flutter on a gentle stream of air. If they will not move easily, support the cheeks lightly with two fingers or choose a hum instead. More pressure is not the goal.
Add one easy note for a few seconds, then rest. The trill and pitch should remain steady enough that you are not fighting either one. Stop before dizziness, discomfort, hoarseness, fatigue, or strain.
Move through a small five-note pattern in the middle of the range. Change the starting pitch only while the previous pattern remains easy. Do not use the trill to force a maximum note.
A smooth trill can provide feedback about airflow and continuity, but it does not diagnose support, fold closure, registration, or tension. A trill stopping in one place has several possible explanations.
After the trill, sing one ordinary phrase and notice whether anything feels or sounds different. Treat “no change” as valid data. Benefits and preferences vary by singer and instruction.
If a lip trill is difficult or unpleasant, humming is a reasonable alternative. The purpose is gentle exploration, not proving that you can complete one fashionable warmup.