The original advised method was to screw a "broom shank" into the top of the stantion through the top joke and pull it up through then pinch it tight with the pinch bolt in the lower yoke.
Not a method I was ever happy with, apart from lacking the required third upper limb I was always thinking of bits of wood floating in the fork oil.
Alternatives were to modify an old fork top nut, grinding off the hexagon bit down so that it pass through the top yoke (like the broom shank) and welding on an extension on the top, then use this in place of the broom shank.
or my preferred method buy a ready made tool, there reasonable priced and make the job easier
best of luck with it, lets know how it goes