Basically you shape the end of a broom handle into a cone/point, not sharp but say 10-15mm across at the sharpest bit. With the springs removed, insert the broom handle pointy end in first. Rest the bottom of the fork leg on the floor (on a block of soft wood or rubber mat to protect it), then tap the broom handle down into the fork leg with a mallet and onto/into the top of the damper rod. The conical end of the broom handle will shape itself to fit the top of the damper rod. You can then use the broom handle to hold the damper rod in place whilst you crack the nut on the bottom.

Another little tip: If you soak the tapered end of the broom handle in water for a few hours before hand, it softens the wood and lets it mould much better to the top of the damper rod and require less of a whack to get it to seat.

You can sort of see the idea (from about 3:45) in this guy's video of a DR650 fork seal change...

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