When I did mine a couple of years back it was, as others have said, one primary at a time, incrementally enlarging the hole to suit each primary.
Lots of masking tape around the current hole.
I also used a large repair washer to run round the new primary with a sharpie pen in the centre so that I got a nice clear line which was about 15mm larger than the primary.
for cutting the hole I initially tried a dremel, but wasn't terribly happy with the noise and vibration in the side skin, so I ended up using a Monodex shear to cut the aluminium, they work better on straight lines to be honest, but it worked fine with care.
If you'd like to borrow my monodex then drop me a blatmail
All the best
Richard