You'll want to use a masonry drill bit that fits the size of the metal rods on your post/stake. It takes some patience but the end result is worth it I think. Be sure to use a slow trickle of water in the hole you are drilling to be more efficient and to extend the life of your drill bit. I "corded" electric drill generally works better too because it has more torque and this tends to take a little while to get through. The thicker the flagstone you are using, the more secure the pole will be because the stone won't allow it to swing back and forth with the snug fit. Another way to speed things along when drilling is to use a nail punch and a hammer to chip away some of the stone as you drill. Just take your time and the results will be worth it.