Use the corner set up as a template for all defending when the ball is hit into the box.
This is the template so understand this first and then adapt this for fee kicks and crosses.
Defending Set Pieces : Free kicks into the box.
On free licks, at the highest level, there is man marking but only if players threaten the goal scoring zone. But as part of this man marking process, defenders get to the goal scoring zone first (they dont let the attackers have a free run onto this zone). By the time the free kick lands in the goal scoring zone, there is a large number of defenders in the zone ready to challenge and block shots.