Take measurements on the inside face of the transom to establish the position of the cutout and drill the pilot hole outwards from there first, otherwise you may end up chopping through the deck, rather than just above it
When you've drilled the pilot hole, start the cut of the main hole with the holesaw from the inside to make a clean cut through the inner face of the transom - don't go all the way through. Do the main cutting with your holesaw from the outside of the transom.
Good luck and some pics on here of your handywork will help other folk in future.