A set of exercises before singing: body relaxation, breath activation, resonance engagement, range exercise.
1) Relaxation: roll shoulders, head, jaw. 2) Breath: inhale to belly, exhale on 'shhh.' 3) Resonance: 'mmm' with lip buzz. 4) Range: glissando from center up and down.
Warm-up prevents injury and improves performance. Like an athlete wouldn't sprint without warming up, a singer shouldn't sing without a warm-up.
5–10 minutes of warm-up before every singing session is the professional standard.