1:Make Sure your are on rooted stock rom with custom recovery instaled.
2:Backup up your all data(contact, sms, etc).
3:Wipe all data and factory setting.
4:Then Processed to rom installation part.

A How to on flashing ICS(Defy Plus Builds)

1:Head over to Stable Recovery from boot menu
2:Wipe Data (Nandroid Backup Before Doing this [optional])
3:Install Zip from SD
4:Select ur ICS(DefyPlus) zip
5:FLash
6:Flash CM9 Kernel
7:Flash Gapps zip
8:Clear Cache and Dalvik Cache

How