I use the same method as Don in Austin but with a refinement. I thread a long bolt through the starnut to get it started and drive the upper pedals flush with the steerer's top edge. Then I remove the bolt, add the top cap and thread the bolt through the top cap and back into the starnut. Mark the bolt about 15 mm above the top cap and use the hole in the top cap as a guide to drive the starnut down the proper distance. That technique always sets the starnut at the right level and perfectly square.