A deck core is a sandwich of fiberglass skins laminated to a foam, balsa or plywood core. In order to "pot" a hole you must first remove a small amount of core from between the skins. I often read of the "bent nail and drill" for this task but it's not a pretty or an easy task. I sometimes question whether the authors and folks recommending this practice have actually ever tried it, but I digress.
While I have chosen to illustrate this using a balsa core you should be aware that most builders do not use foam or balsa directly beneath high load deck hardware. More often than not you'll be dealing with marine grade plywood under deck fittings. Plywood can handle the compression loads far better than either balsa or foam cores and this is why it's laminated into the deck beneath deck hardware. Plywood is hard to carve out, comparatively speaking, and one more reason why the bent nail trick is not as easy as some would make it seem.
The easiest way to remove core from between two skins, that I have found, is with a Dremel. I also advise the use of the "magic wand" as I like to call it. This flexible extension gives you artist like finesse and makes the job of removing core material significantly easier. Dremels are cheap these days and I can't imagine owning a boat without one especially when potting deck core penetrations.