To get the holes to be a just right fit for the lubed patch and ball, I use a long tapered reamer going to the centre from both sides of the board. You want it tight enough to hold the balls without fear of them falling out, yet not so tight that the lubricaton soaks entirely into the wood, and you can easily push them into the muzzle with a starter, or the ramrod. So, drill the holes, for example, a fifty cal. you can start with a 1/2" drill bit. You won't be able to seat a patched ball into that hole, so ream it out from both sides until you get a nice fit. A lot depends upon how thick a patch you use, so I can't be more specific than that.