If you've got it rooted already that makes it a lot easier.
Edit: Backing up your stuff is probably a good idea! Do it however you want, Titanium back (Stored on the phone) or Samsung Kies (considering you're running stock)
1. Download the ROM you want. Best place to look for them is XDA, choose what you want and download it on your phone, or onto your PC and then transfer it to your phones storage.
2. Make sure you download any additional patches or packages required, note: some do require extras!
3. Turn your phone off
4. Start up in Recovery Mode (Volume Up + Home + Power at the same time until GSII logo)
5. Do a "Wipe Data / Factory Reset".
6. Go to Advanced > "Wipe Dalvik".
7. Go back to the main menu, Mounts and Storage > "Format System".
8. Go back to the main menu > Install Zip From SD Card.
9. Then select "From Internal" or "From SD Card", depending on where you put, then go through the folders till you find it. Install the ROM's zip file.
10. Repeat step 9 for any patches or additional packages.
11. Go back to the main menu, "Reboot System Now".