The scalp is cut in a crescent shape and peeled open for the operation, and then folded back down and sewn back in place, so the hole is covered by the scalp.

Apparently it's not as bad as you'd think. I can't recall her name right now, but Michael Pollan talks about a British socialite that trepanated her own skull in his book 'How to change your mind'. Apparently she did that and went straight to a party, and she's alive to this day. I think there's even a video of her doing it online.

The body's amazing at recovering from all kinds of stuff.