Ok I agree with the guys above around cost/damage but for me there is another element - the fun of the project and an opportunity to learn something.
I really enjoy getting my Dremel tool out and modding my PC, especially when it goes right and looks good. Which brings me on to my next suggestion - get an old unwanted pc case and have a test run on that first. You will make mistakes if this is your first time cutting into a PC case and equally if its your first time with a hole saw or dremel tool.
To do it, in short, work out where you need your hole. measure this 3 times to be sure you got it right. Then cover the area in masking tape so you cant scratch the case while working. To measure the exact hole, the easiest way is to buy a £2 120mm fan grill and draw around that - simple.
I personally use a Dremel rotory tool, cutting the same way they do on many of the YouTube guides that are out there. I'll leave you to search for those as seeing it being done is most useful. Top tip - don't force the tool, let the tool do the work and take your time.
Once the hole is cut, you can give it a nice edge with soem rubber U-channel sold at many PC modding stores. drill the holes for your fan screws (again, use the grill to see where to drill) and dont push down to hard or you will make an indent on your case.
Thats pretty high level but it's what I do and the results are good if you just take your time. Measure 3+ times - grab a coffee - look over things for any obvious errors - and then cut.
Good luck