On hands and knees, hands under shoulders, knees under hips. Inhale, drop your belly, lift chest and tailbone (cow). Exhale, round your spine toward the ceiling, tuck chin and pelvis (cat). Move slowly, matched to your breath — this is a warm-up, not a stretch to hold.
Lie on your back, arms reaching straight up over your chest, knees bent 90° over your hips. Slowly lower one arm behind you and the opposite leg toward the floor at the same time, keeping your lower back pressed flat against the floor. Return to start, alternate sides. If your back arches off the floor, you've gone too far — smaller range with a flat back beats bigger range with an arched one.
On hands and knees, extend one arm straight forward and the opposite leg straight back at the same time, keeping hips and shoulders level — don't let your torso rotate. Hold briefly, return with control, switch sides. Move slow enough that a cup of water on your lower back wouldn't spill.
Forearms on the floor, elbows under shoulders, body in a straight line from head to heels. Squeeze your glutes and brace your abs like you're about to be poked in the stomach. Don't let your hips sag or pike up. A shorter plank with a straight line beats a longer one with sagging hips.
Kneel, sit back onto your heels, fold forward with arms extended (or resting alongside your body), forehead toward the floor. Breathe slowly and let your lower back lengthen. This is your cue that the day is over — don't rush out of it.