It should be M14x1.25mm.
Rule of thumb for all tapping drill sizes is; Nominal bolt dia. minus thread pitch.
In this case 14mm minus 1.25mm.Tapping drill size; 12.75mm.
Obviously, use a drill press to ensure you get the holes square, with the caliper firmly clamped. Use a tapping compound or grease to lubricate the drill, not too fast, take your time.
When tapping, use a tapping compound or grease to lubricate the tap, you don't want it to bind or break. Don't be shy, use plenty, it wipes off at the end. Ensure you are using a Taper or Intermediate tap; a bottoming tap will be almost impossible to start. (or just buy a complete tap set; it comes with all three).
Take your time to ensure the tap is straight and square once you start a tap crooked, it can be difficult to straighten. For every full turn on the tap, back off 1/2 a turn to break the chip, when you wind it back, it will go tight, then the chip will break, then release. If you don't do this the thread can start to tear, and feel rough; you want a nice smooth finish. Use a proper tap wrench, not a shifting spanner; it will be much easier to ensure the tap is square.
I hope I haven't oversimplified this, not trying to treat you like an idiot, just wrote for novice use.
Good luck, Dale