I did mine in situ, using the same chain drilling technique.
Take it slowly to be sure the bit doesn't slip.
I also removed the gear lever surround first, so I could feel underneath to make sure there was nothing I could damage, and also as you can then feel the reduced thickness where the trim is marked out for the button.
I finished off with a rat-tail file to smooth all the edges. If you drill the hole slightly too small you can then carefully file a bit and keep trial fitting the button until it fits snugly, rather than run the risk of making the hole too big.
