Start small and increase sizes. You want to wind up with a drill size that equals the tap diameter measured across at the bottom (waist)of the threads (not at the tip but on the body of the tap). For instance, a 3/8 tap will need approx. 9/32 drill. The tap may have the needed drill size printed or stamped on it. The tip should go into the hole enough to just start cutting threads. Cut about a quarter to a half turn then back out a bit to clean the cuttings. Take your time. The cover metal is somewhat soft. Make sure the tap is straight as you start the cut, it will guide itself after it starts cutting threads. Use some thin cutting oil or WD-30. Happy tappy...