For those of you who’ve rooted your Galaxy S2 i9100 using the new method (that doesn’t install ClockworkMod Recovery), you will need to install ClockworkMod Recovery after rooting if you want to install custom ROMs. (If you want to just use your stock rooted ROM, you don’t need to do this.)
Step 1. Put your Galaxy S2 into download mode by holding down Volume Down, Home, and Power buttons together until you see the warning screen like below:
Step 2. Next, hit Volume Up button then connect your phone to your computer via microUSB cable.
Step 3. Next, download GalaxyS2Recovery.zip and unzip, you should find two files like shown below.
Step 4. Verify you have drivers installed by going to Device Manager, you should get something like below, SAMSUNG Mobile USB CDC Composite Device.
If you don’t, download Samsung Kies software and install it. Then re-plug your Galaxy S2’s microUSB out and in.