Flash gapps*.zip. You must flash gapps after every upgrade to CM9 since /system is formatted, wiping your previous add-ons.
After you setup your Google account, reinstall the three apps from Market in order to restore most of your apps, call log and SMS history.
Warning: Do not restore backup data of system apps!
If you restore a bad backup with cached settings in telephony.db in data/data/com.*.*.telephony it can break MMS.
It seems that restoring backups can also break Calendar sync.
Upgrades from previous versions of CM9 are the same process as install, except you do not need to wipe anything. Contrary to popular belief, you do not need to even wipe cache and dalvik-cache when upgrading between CM9 versions. You only need to wipe if you are doing a major upgrade, switching to a different ROM entirely, or attempting to fix bugs caused by bad backups or corrupt app data.
Epic Touch CM Team for the install instructions
All SGS+ users that helped with testing and bug reports
ivendor and mirred for help with the camera HAL
Finally it has an own topic here in the I9001 section! Thanks for your great work and keep it up! I really want to use ICS on my phone but I'm a noob in programming, so I can not help... But I look this topic every day maybe it has the camera working... Now, in this state I won't install it (but I really want it!!!) but when you say the camera is working perfectly, I will immediatly install this rom!
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