My personal favorite way can be done with just a hacksaw blade, some tape and a metal file.
Wrap some tape around one end of the saw blade, enough to hold, as well as some on whatever you are cutting, lay the tape in the outline and then using a ruler for a straight edge mark the exact lines. Remember you can always take a little more away so aim a little bit shy.
Once you get the lines slowly cut the two sides and then angle across the middle. You can then saw out the remainders and file any imperfections out.
REMEMBER, very important, go slow and do not rush. Take your time and be sure. Yes you will cut yourself, but a bit less with practice, yes you will err, but again a bit less with practice.
Masking tape works best.
Also if the hole needs to in the middle and not edge, a couple well placed hole drilled into it and a smaller metal file will help