Step 1: Measure and mark
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.