In many first classes the instructor says to breathe into the stretch, and it isn't clear what that is supposed to make you do differently. Most people nod along and ignore the phrase for weeks, assuming it is just something teachers say.
Your first day
Starting takes ten minutes and four moves, breathing in and out through the nose throughout.
Begin in child's pose, forehead down, arms extended, and breathe there for five slow breaths. Move to hands and knees and alternate cat and cow with the breath, ten rounds, inhaling as the belly drops, exhaling as the spine rounds. Push back into downward dog, hips lifted into an upside-down V, and hold for five breaths, knees bent as much as they need to be. Walk the feet up to the hands and fold forward, letting the knees stay soft.
If a pose gives sharp or pinching pain rather than a stretch, ease out of it. Nobody's spine is straight coming out of a long day, so don't force the fold. A bent-knee fold with a long back beats a straight-leg fold with a rounded one every time.
You're done when you can move through all four with steady breathing, even if none of the poses go deep.
The thing most people miss
Flexibility isn't what limits most beginners, attention to the breath is. Hold a pose while your breathing goes ragged or stops and the body tends to tighten against the stretch, so the pose feels harder than it is.
Matching movement to breath gives the pose a rhythm to follow instead of a clock to watch: inhale as the belly drops in cat-cow, exhale as the spine rounds. That is what breathe into the stretch means. Let the exhale lengthen you into the pose.
How you'll know it's working
The tell isn't a deeper stretch, it's a longer breath. Over a few weeks, notice how many breaths you can hold downward dog for before your arms shake. That number usually rises without any deliberate strength work.
Sun salutations link these same four shapes into one continuous sequence, with a movement on every breath. They are the natural next step once each shape feels familiar.