I've done a couple now. As ECR says, slowly and a bit at a time. Tools I used were a junior hack saw blade with the pin knocked out and gaffa taped to hold it, orientated to cut on the pull stroke. Some small tin snips (I have a set of jeweller curved snips). Half round file and various grades of wet and dry to finish the edge. Sharpie and plastic ruler. If painted be wary of using masking tape up to the very edge of the cut as you dont want to peel the paint off with the tape (I've known someone peel the chrome off a headlight they were altering!!). Reseal the ally edges with paint once you're happy its the right size. My technique was not pretty mid task but looked good once complete. Use the saw blade to rough out to the desired size - this often meant cutting little triangles of ally, as the blade doesnt cut curves very well, then blend in the cuts with the file. The filing needs to be done gently with light pressure. The ally is quite soft and can distort easily. On straight sections I used the flat of the file at an angled to get a reasonable length of file along the cut edge and file it straight. I've read about others using an emery flap wheel to enlarge the hole, which I did when increasing the height of the steering rack holes by a few MM. I think there is too much material to take out when doing the exhaust aperture. Ian