This skull is made of resin which despite feeling rock solid is actually really easy to drill. Ideally I would have used a Drill press but I don't have access to one so I had to do it by hand. Start quite slowly and stop if you think the bit is getting too hot. This bit needs to be really neat to look good so take your time and don't drill too deep.
The flat bit will give you a nice starting point for the smaller drill bit. Use a regular HSS bit to make the smaller hole otherwise it might pop out of the top of the skull. Go slowly to avoid drilling right through.
Neaten up the edges with a small needle file or sand paper.