Once you have found your seat and sit relaxed, your breath is your “anchor” during your formal practice. And mind you, when you integrate your practise in your daily life, the awareness of your breath is also your anchor.
While breathing in your breath expands your whole body, and this you experience especially in your torso.
While breathing out you experience the opposite of expansion, letting go and body volume getting lesser.
Follow for a couple of minutes this expansion and letting go. In you own rhythm , do not change anything, just be aware.
Just follow the breath.