Follow this progression from gentle to more demanding:

• Lip trills (2 min) — blow air through loosely closed lips to create a buzzing trill. This is the gentlest way to engage the vocal cords while releasing tension in the lips and jaw.
• Humming scales (1 min) — hum up and down your range gently, feeling vibration in your face and chest.
• Sirens (1 min) — slide smoothly from your lowest to your highest note and back, like an ambulance siren. This stretches the vocal range.
• Tongue twisters (1 min) — "Red leather yellow leather" and "Unique New York" wake up the articulation muscles.

The key principle is gradual escalation.