You MUST have root beofre flashing GalnetMIUI Chameleon RC1. If you do not have root, please follow this guide (thanks to Chainfire) : http://forum.xda-developers.com/show....php?t=1103399
Step 1. Download GalnetMIUI for Galaxy S2
Step 2. Now connect i9100 to PC with a USB cable >> copy
over the GalnetMIUI .zip file to the root SD card >> then
disconnect your phone from PC.
Step 3. Download and install ROM Manager app
Step 4. Now run ROM Manager app >> tap on Flash
ClockworkMod Recovery >> choose Samsung Galaxy S II
>> now your phone will reboot.
Step 5. Once the GS2 is rebooted >> launch ROM Manager app >> tap on Reboot Into Recovery.
Step 6. Your GS2 will reboot again and will enter ClockworkMod Recovery
Mode. Now use volume keys to navigate >> scroll down to
backup and restore and make a backup of your smartphone’s current state.
Step 7. Once the backup process is finished >> scroll down
to wipe data/factory reset >> scroll down to advanced
using volume keys/home button >> wipe Dalvik cache.
Step 8. Scroll down to install MIUI zip file from sdcard
>> choose zip from sdcard >>
Chameleon-RC1-i9100. Wait till it be flashed completely
Step 9. Finally, reboot again via ClockWorkMod recovery
Step 10. Now your Galaxy S2 should run GalnetMIUI