Get there first and dive in. Read the low cross, get across the defender, and be brave to head it in before he clears it.
As the base, but a defender contests the low ball. Emphasise reading it early, getting across the defender, and brave, well-timed contact to head it in first. Rotate the roles.
Fork it into your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block.