mate its simple as unbolting ur old gaurds and take them off, then bolting the new gtr ones on
the only thing u will have to change is the inner gaurd plastic, so make sure you have a gtr one first.
there is no complication to this at all, and if you know where all the bolts are located to remove the original gaurd your fine
and if you dont know where they are take your time and take the bolts you can see out first then try and move it and see where there is still a bolt or where it isnt moving and take it easy and you will get it.
to help you there are 5 or so bolts in the engine bay, 2 in the door area and one behind the indicator, and some in around the wheel arch. and u will need to move the side skirt to get to one or two aswell
i think thats it but yeh take ur time and you wil learn along the way.
pm me if you want pics on where the bolts are located.