What's the easiest way to insert a star nut into a steerer tube?
The easiest solution is to use the right tool to make sure the star nut is aligned and set to the right depth in the steerer tube. I'd really recommend taking this to a shop and asking to borrow the proper tool. I own one, even though it only gets used once or twice per year. I've tended to go more towards compression plugs instead of the old star-fangled nut. If you're looking for an alternative to using the proper tool, you can thread a bolt in the nut and tap carefully with a hammer, or perhaps use a dowel or socket of the appropriate diameter instad of a bolt.