Why do you want to fit these? and what car are you fitting them to? If it is just to tart up an old Nova or Escort or similar, to make it look like its got something interesting under the bonnet, then it probably just look naff, and in that case I cant see why you would need to cut holes in the bonnet, just spray them and whack them on! They will never look as if they belong there, so it hardly matters how you do it. Remember that they will create extra drag and actually adversly affect the cars perfomance! If you have done some tuning work, especially fitting a turbo, and you actually need some more under bonnet cooling then I guess you will need to cut the holes, but you still need to get the things in the right place airodynamically, not always easy. Personally I would think about other options, like fitting an oil cooler, or an extra cooling fan, both will be effective without adding the dreaded 'boy-racer' look to your car!