Mark the spot on the sink where the center of the hole will be, using a permanent marker. Fit a drill with a 3/8-inch carbide bit.
Press the drill bit into the mark and begin drilling. Apply steady, heavy pressure to the drill until the bit goes all the way through. This 3/8-inch hole is a guide hole.
Remove the 3/8-inch drill bit from the drill. Replace it with a 1/2-inch drill bit.
Insert the drill bit into the guide hole and turn on the drill. Move the drill bit down through the guide hole until it widens to 1/2 inch and the bit goes in and out of the hole easily. Remove the drill bit.