I found the process relatively simple, remove the old exhaust and offer up the new primaries, one of them will pass through the existing hole. then set out the location of the other primaries relative to this and cut to a tight hole, offering up the next primary as the hole increases. once all fit through the tight hole, remove them all, draw the shape of the finished hole you like (allowing some clearance for engine movement) and "tidy up" the hole with a flapper wheel fitted to an electric drill. I used jar lids as a template for marking the radius sections of the holes, and have a look at images of other cars to understand the shape required. It only took a few hours.