Step 1. Download the appropriate CWM Recovery ZIP file for your Galaxy S2 GT-i9100 or SGH-i777. (If for some reason you do not have a custom recovery installed already on your phone, you can also grab the TAR version and flash it using ODIN.)
Put the ZIP file on your phone and also put the Android 4.4 KitKat ROM you want to install on your phone.
Power off your phone and reboot into your recovery by holding down Volume Up, Center Home, and Power buttons together for about 20 seconds.
Step 2. Once in recovery, choose “install zip” for CWM or choose “Install” for TWRP and browse your file system to install the ZIP file you downloaded earlier.
*Note – If you see CWM Recovery v184.108.40.206 or higher at this point, you may skip to Step 4.