easiest way is with a star nut setter. This tool allows u to set it in straight which will have you a lot of headaches in later on. If i'm home i use a top cap bolt and a hammer and drive it in about 1/4" below my cut.

The spacers you use are all a personal preference on how high or how low you want your bars to be. It's a good idea to set up your front end first(new headset, fork, stem, bars, and wheel) to get an idea of where your bars to be. That will give you an idea of how many spacers you will use. It's also a good idea to get a couple spacers of different widths, this was you can play around with the length of your steerer tube.