download these 2 apps. odin3 and cf-root (obviously be sure you get the latest versions of them of course...)

odin3 is installed on your pc.

you also need to download and install the official samsung Kies on your pc.

then turn off your phone. then press hold the power button + menu button + the bottom part of the volume rocker together to start the phone in data receiver mode.

then plug into your pc, and wait for kies to detect the phone. When it's detected, run odin3, and follow the steps in the guide which i believe is to flash the cf-root file using odin3.

don't turn off power or unplug during the process or your phone will brick.

when all done, your phone is now rooted. I recommend you browse the settings and options of phone to understand how it works before you decide to upgrade rom to Wanamlite.

Once done, then download wanamlite zip file, and put onto your S3 e.g. into the download folder. Use airdroid to do this, it's a free android app on googleplay store via your S3.

when done, reboot your phone by press holding the top half of the volume rocker + menu button (the middle one) + the power button all simultaneous. Wait until you've booted into the recovery mode before you release the buttons.

First thing to do after rooting the phone, but before upgrading to a new rom
1. factory reset
2. clear cache
3. clear dalvik

Once done, only then do you select install zip, browse internal sd to the downloader folder, and select the wanamlite version 2.2 zip file.

The screen will then say it's installing. when it says reboot, reboot as instructed. during start up it will install the rom.

Configure your device etc and your done :}
i know reading a bunch of text is confusing, hence please refer to the youtube video guides which pretty much is the same as what i've explained