What kind of case do you have, and have you done any initial measurements? If your serious about doing it, and absolutely want 140mm, then the best place to start is with a 140mm fan grill, like you find on most PSUs...the mounting holes and size of the vent holes are already there. You can pretty much lay the fan-grill on the planned and taped (using low tack masking tape or painters tape) off section, dot the screw holes, the very center of the grill for a bit guide, and outline the circular fan grill section with a sharpie.
Here is a guide for adding a side vent fan of the 140mm type. http://forums.overclockersclub.com/?showtopic=175283
Keep in mind that if you decide to use a hole-saw attachment(5" for 140mm), you will need an 18v or higher (recommended) drill (if you use a cordless drill), to get the job done, especially on sheet metal, SECC steel, etc. I've done it , as well as one on the plexiglass side windows in a couple of cases, but I chose 120mm.
Make sure you wear eye protection, keep your drill as straight and even as possible, make sure that what you are working on is level and even, and held firmly in place via C-clamps or other means(you dont want it fly around when the pressure is on, and watch the metal (and smell it, specifically for a burning plastic smell). You dont want it to get too hot as it will warp, so if you start smelling that, it's best to back off, wet it with a wet drag and wait about 30-60 seconds. You could do what most do and use water, but then you have to make sure to dry it off afterward (what doesnt evaporate) and sand the inner edges when you are done. I use a dremmel tool and attachment to take care of edges.