Measure the location of the hole. If possible, mark exactly where the hole needs to be drilled.
Choose the drill size appropriate to the material being drilled and the tool being used. Use common sense. Trying to directly cut a 3/8 inch diameter hole in stainless steel with a hand drill is not the wisest idea. Start with small diameters and work up. If you?re not confident in your common sense, try a handbook such as Machinery's Handbook
Use this chart
to determine what size to drill for a bolt or screw.