Hi Jonathan,
I generally screw about a 40mm long 6mm (or is it 5mm?) Allen screw into the star nut, place on top of the steerer and tap it in with a hammer on the head of the screw.
You need to give a fair old whack to initially get it sprung into the steerer.
Once set to roughly the correct depth I generally tap the screw around a bit so that the screw is sitting nice and straight up the centre, so that I know that when I come to fit the screw through the top cap it will engage OK.
Not very scientific I know but it works for me! I guess its possible that you could break bits off the star nut with the initial hit (it has never happened to me) so this should probably come with a health warning - eye protection might be a good idea.
Another way might be a long piece of metric threaded rod in through the bottom of the steerer with a small piece of plate (or a top cap?) and a hexagon nut to pull the star nut in. Could be a bit laborious due to the fact that the fork blades will get in the way of full rotations of the spanner, although a deep socket might speed up at least the first part of the process.
Have fun!
David