in cwm. mount and storage > format system
These are the step to install Rootbox ICS 4.0.4:
Recommended Installation Steps
Coming from another Custom ROM:
Wipe Data/Factory Reset (This does not affect your Internal/External storage)
Format /System (This does not affect your Internal/External storage)
Flash ROM
Follow Aroma Instructions (Uncheck Reboot Device when Aroma Installation is complete)
Wipe Cache
Wipe Dalvik Cache
Reboot
Coming from Stock Samsung ROM:
Root Phone - Get CWM Recovery (Refer to How to root your Samsung Galaxy S2)
Wipe Data/Factory Reset (This does not affect your Internal/External storage)
Format /System (This does not affect your Internal/External storage)
Flash ROM
Follow Aroma Instructions (Uncheck Reboot Device when Aroma Installation is complete)
Wipe Cache
Wipe Dalvik Cache
Reboot
But i'm wondering, how to format /system?
One more.. the file is zip file.. Use CWM to flash?