How do I nail T and G vertical siding?

It can be secret nailed - nails punched into/through the groove, at an angle, as each board goes up. Bbecause each board is only held at the edges, if the timber is not great quality, it will tend to move and never be flat. As the advertising geeks say though - if you can't get rid of a problem, make it a feature - you could use brass or copper nails or screws and make them a feature - but positioning would be critical.