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How to tap a hole

1. Look up the tap drill. Use the finder or the chart. Don't guess: one size too small is the most common reason taps break.
2. Mark and centre-punch the hole, so the drill doesn't wander.
3. Drill square to the surface. A drill press or drill guide helps. In metal, use cutting oil and moderate speed.
4. Chamfer the hole slightly with a countersink, so the tap starts cleanly and the first thread doesn't burr.
5. Start the tap square. Use a tap wrench, check from two directions, and put light pressure on for the first turn or two only.
6. Turn and break the chip. In steel, go forward about one turn and back half a turn. Use plenty of cutting fluid.
7. Blind holes: drill deeper than the thread you need and clear the chips out often. Finish with a bottoming tap if you need full threads near the bottom.

Safety

Wear eye protection, clamp the workpiece, and never hold small parts by hand under a drill press. Broken taps are hard and brittle, so their pieces can fly off.