Tis easy enough.
Access is a bit tight.
If its the cars 2nd belt change then the get new tensioner and idlers. Chaneg the water pump if you want but that can be left as its not driven by the cambelt (I am assuming of course your Kelly with the williams and the C.N.U.T logo on RSC).
Anyway basically off with the top mount to free up some space turn the crank over with a 19mm socket till TDC is found, get the crank bolt out, loosen the tensioner old belt off. Replace the idlers and tensioner on with new belt and tension.
You need a T30 bit for the lower idler pulley and 16mm spanner for the tensioner. Assumign you get a proper 16V tensioner it will have a little tang on it for the tensioning screw. Take the wiring P clip off and screw in a rocker cover bolt into the hole to tension it.