the haynes manual does actually cover it very well, main point to remember is to tension the front pulley then the rear one, and re-check a few times to make sure the timing is correct.
this has quite an effect on performance on these engines as they're quite sensitive to cam timing, I remember when Paul put my 405 on the RR down south it was only running 140 ish bhp, but a big chunk of torque, paul changed the belt a week before I bought the car, and I've had it on 2 different RR's at over 160 bhp, with healthy torque as well