At the local opp shop I gathered a collection of different sizes of cups, saucers and bowls. I wanted them to match, but I also bought some extra to practise with.
You have to drill quite long and hard. I was surprised at how tough and at the same time fragile pottery can be. And how different one kind of pottery is from the next.
Tip: the drill and the cup can get hot. Rinsing the cup with cold water cools it down and leaving it undried makes the drilling easier. I broke a few cups. Luckily not the ones I really liked.