Doing this myself. Just involves shutting off power to the cooker, unscewing and removing the sides, unscrewing the now visable screws either side at front of the cooker holding in the hob and also the three screws under front of the hob. You need to buy the Halaogen Ring specifc to your make / model of cooker. Did you look at the back of the cooker and locate the serial number? This is important as creda may weell end up sending you the wrong replacement ring otherwise as there are at least two types.
Chris Mulhall, October 2008