Now that I'm at a computer and can type... I had a stem riser from a previous bike, why I said leave the steerer tube uncut, that I used to drive the star nut. I threaded the star nut into the riser just as I would attach it to a steerer on a bike. I eyeballed the depth I wanted to drive in the star nut, very conservatively, then held the steerer tube and riser in my left hand and tapped on the stem cap with a rubber mallet with the right, it really didn't take much force to tap it in start light and increase force until it starts going in. In total it took me less than two minutes to seat the star nut and it went in straight as an arrow. Maybe I got lucky but in any case the stem riser will be used for future star nut installations.

You really should consider leaving your steerer tube uncut as it won't take much more effort to cut it in the future than it will now and once it's cut it's cut. I know I regret how I cut my first steerer tube, hence the reason I own a stem riser.