If it's not pre-drilled and you don't have experience doing the drilling, you basically have two options.
1. Contact your LFS
and see if they can recommend a glass shop somewhere that can do it for you. It's usually not that expensive, but if the glass company messes up and breaks the glass, they usually have a policy that if it breaks your out of luck, no reimbursement.
2. Use one of the EShopps over flows. I was in the same situation and chose to go this route instead of risking the tank and I haven't regretted it. I chose the PF-300 for my 55 gal
and it works like a charm, plus if any fish get caught in it they are easier to catch, where as a corner overflow is tough if you have big hands.