You can find a detailed guide with screenshots and all here. It's done with odin.
Step 1: Install Odin v1.85 on your computer then download and unrar
the Lite'ning Rom rar file.
Step 2: Make sure that your phone is in USB debugging mode: Settings
-> Applications -> Development -> USB debugging.
Step 3: Reboot your phone into download mode. To do so: Turn the
device off, then power it on again by pressing and holding the Volume
Down + Home + Power on simultaneously.
Step 4: Start Odin.
Step 5: Connect the phone to your computer using the USB cable.
Step 6: Wait a few seconds, the Odin screen will show that a device is
now connected... Added!!
Make sure that in Odin nothing is checked, except the "Auto reboot" and "F. Reset Time" checkboxes.
Press the "PDA" button, and select the md5 file.
Press "Start". Odin will now flash the Lite'ning Rom, after flashing your phone should reboot.
Step 7: Done! Now you have a fully rooted SGS II with the Lite'ning