Slow breathing with the belly, without strain.
Lie down or sit. One hand on the chest, one on the belly.
Feel gravity: let your weight drop into your feet or the seat. Feel your weight being held by the seat or the ground.
Inhale: the belly rises first, then the chest.
Exhale: the belly falls first; at the end, gently draw it in.
No forced deep breaths: simple, steady, slow.
The hand on the chest hardly moves.