A simple but effective drill to shoot free throws with concentration.

8 or more players 2 or more balls.

A player shoots 2 free throws in a row.

If he misses 1 he runs to the mid line and back to the base line.
If he misses 2 he runs to the mid line  to the base line to the end line and back to the base line.
With no missed balls just help with rebounding.

In this case 1 has missed 1 shot and 5 has missed 2 shots.

Shoot 5 or more sets of 2 free throws.