Drill a Hole, and Smooth the Edges
Using acordless drill and a stainless steel bit, drill a 3/4" hole into the sink (Image 1).
A slug buster can also be used to cut the hole. To do so, take the cutting wheal off the slug buster (Image 2). Place the slug buster into the hole, then reattach the cutting wheel underneath. Hand tighten the nut on the top side of the slug buster, then use a wrench to continue tightening until you hear a 'pop' (Image 3). Remove the slug buster.
Use an emery cloth to smooth the edges
Safety Note: Always wear safety glasses when cutting metal.
Note: If the sink is cast iron, make the cut into the countertop near the sink using a hole-saw (Image 4).