When drilling larger holes always start with a smaller bit and work your way up and make sure the hole is it least half the size of the bigger drill or it will grab the job and mess the hole up. Make sure the job is well secured. For small drill bits you use a fast drilling speed and for large ones slower speeds and very light pressure. If you are sharpening your own bits make sure you don't make the angle to steep or the leading edge too high in comparison to the trailing edge.
For very large holes 15, 18, 20, 25, 32mm etc buy some hole saws and a selection of half round second cut files. I have one who's radius exactly fits a 32mm hole, perfect for large industrial push buttons
For large rectangular holes mark out the opening with a scribe (nothing worse than getting half way through to find your pencil outline has rubbed off).
Drill 2 large holes in opposite corners large enough to fit a hacksaw blade. Make sure you don't get to close to the edges of the hole as its better to have to file some metal out than drilling over the edge.
Holding the hacksaw blade with your hand as close to the job as possible (you can actually buy special hacksaws that allow the blade to protrude out one end) and cut from each hole to the other corners, again, don't go to close to the desired edge.
With a nice sharp flat 2nd cut file, file off the very large lumps normally and get each edge as straight and parallel to the desired edge as possible. When you start getting close start using the "draw file" technique (see figure 6
hereWhen using files, hacksaws or pretty much any cutting tool on aluminium, stop often and clean the aluminium out of the tool. For files you can use a file card or a brush with hard bristles( see figure 5 in the above link)