Complex sentence: In home canning, food is packed into the jar, leaving some empty "head space" between the level of food and the top of the jar, then the lid is placed on top of the jar with the integral rubber seal resting on the rim.
Split into simpler sentences: In home canning, food is packed into the jar, leaving some empty "head space" between the level of food and the top of the jar. The lid is placed on top of the jar with the integral rubber seal resting on the rim.