How do I hold the crankshaft while tightening the vibration damper center nut?

Put the timing belt on, tensioner in place and unlocked. That should place enough friction to safely tighten the 4 bolts and the center bolt.

There is a $50 tool to hold the vibration damper, but it shouldn't be needed.

Shops in a hurry use an impact wrench, just a short burst. NOT recommended.
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Keeping it running is better than buying new

So, I don't have to torque the center nut to 180nm, & the 4 bolts to 18 ftlb. then another 30 degrees? That would make the job much easier (& cheaper).
Marlin

The torque values aren't that great, the connected timing belt should hold the harmonic balancer in place. MUCH easier than trying to remove the center bolt, which tends to get rusty and balky.
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Keeping it running is better than buying new