Well - there are a few different ways to do this.

One is to use a tool called a Dremel or Rotozip. These use a cutting wheel and require some patience/time to get the hole just right.

An easier way to make a blowhole is to find someone with a holesaw that is the right size - or purchase a holesaw from somewhere like http://www.caseetc.com

You put the holesaw on a drill or drill press and cut out the hole - this is the easiest way to do it.

I prefer to cut out my hole from the back of the case panel - if you choose to do it from the front then tape around the hole first so you don't scratch the finish on your case.