RE: How to tap metal
One thing no-one has commented on is that you need a starter tap. You will notice on some taps that the threads start part way up the flutes. This is to help start the tap into the hole and automatically square it to the hole.
The other taps are called bottom taps or plug taps and the threads start nearer to the end of the tap. You shoulden't need one of these as you will probably drill all the way through your mounts.
The recommended drill size is usually marked on the side of the tap allong with the thread size.
Use a small tap handle. You really do need to feel what the tap is doing inside the hole.
It is also a good idea to use cutting fluid when you drill the holes as well as when you tap the threads. The primary purpose of cutting fluid is to pull the heat away from the cutting edges.
Don't forget to debur the hole before you run the tap in. You can use a drill bit that is larger than the hole and using your fingers, twist it on the bur to cut it off.