Warning: For GT-i9100 and SGH-i777, installing CWM or TWRP recovery also installs kernel. This is only recommended to install when installing a new custom ROM. Installing CWM/TWRP recovery will most likely break the ROM you are running currently as it overwrites the ROM’s kernel.
To use, install CWM or TWRP recovery, then reboot into recovery, and install the ROM as usual. See How to Install Android 4.4 KitKat ROM on Galaxy S2!CWM Recovery for Galaxy S2 GT-i9100