Have you done a search? There are loads of threads about this!
You'll need a KKL lead, either one with colour coded adaptors to switch the pins, or the switchable type like this http://www.ebay.co.uk/itm/vag-kkl-40...#ht_1706wt_847
(this is the one I have). Download the latest FTDI Virtual COM port (VCP) drivers and the free copies of MultiECUScan & DnECUDiag*(was AlfaDiag). You'll need to run the software as an administrator (different proceedures depending on Windows Version, do a search) and turn off all firewalls, so make sure you turn your WiFi connection off to stop attacks while doing this. If you are lucky the software will work first time, if not it will take some tinkering and cursing.
BTW You need to remove the seatbelt pre-tensioner connector and replace it with a choc block or a new connector to stop the light coming on again; especially if like me you take kids in the back who stick they're feet under your seat.