Instead of attacking normally with a ranged weapon, attack each target in a straight line out to your ranged weapon’s maximum range.
These attacks occur in order starting with the enemy closest to you.

You cannot combine any abilities which allow you to attack extra targets with Piercing Shot, including Volley.

When you miss with a ranged weapon and there is a straight line from you to the target you missed to another target within that weapon’s maximum range, you may redirect your attack at this other target.
Roll a new roll to hit this new target.

You may use Line Them Up again on another target if Line Them Up misses.

You cannot use Line Them Up with an ability which attacks extra targets.
(ex: Piercing Shot (you cannot attack the same target multiple times with one shot))
(ex: Volley (you cannot attack one target many times by lining up allies and intentionally missing them))

Line Them Up redirects an attack. It is not another attack (though different circumstantial bonuses or penalties may or may not apply).