Space recovery in ASM

Hi, 

Actually we are having 4 diskgroups in which total 49 disks are there.Now we want to remove around 20 disks ( 20x200 = 4000 GB) from the diskgroups.

  For removing these disks,

1. First, I need to drop the disk using ALTER DISKGROUP <name>  DROP DISK <name>; Is this correct way?

2. Need to check asm_operation view and monitor the rebalancing. correct?

3.Need to check the status column of asm_disk view, once the status changed to drop, then we can physically remove the disk. Correct?

Please correct me If I am wrong in any of the above steps.

And also Please suggest a way to drop all the 20 disks at the same time.