A simple way to "wear-in" the edges of fingerboards is to do it by hand, literally. Wrap a strip of 180 grit sandpaper around two fingers, and use your fingers as the sanding block. Rub along the length of the fingerboard edges, keeping those fingers at a 45 degree angle in relation to the fingerboard surface. You'll be bumping up over the ends of the frets, rounding them off too. Don't let the sandpaper scratch the top surface of the frets; only the ends.
The effect of sanding like this is very much like natural years of wear. It rounds the edge of the fingerboard and slightly scoops between frets. After the 180 grit to rough out the amount of rounding and scooping, go over it with 220, 400, and 800 grit paper. From there, I'll usually polish the edge of the fingerboard with gray and then white Scotchbrite. But, that depends on what you plan to do with finishing the neck. The ends of the frets can be re-polished with steel wool or one of the rubberized abrasive disks in a Dremel tool.
If you are doing this on multiple necks and don't want to wear out those fingers, the cool special tool is a "Combi-Flap Wheel". A Flap Wheel is a small drum of flaps of sandpaper, all glued to a center hub with a shaft. You chuck it up in an electric drill and sand things with it. The Combi-Flap Wheel is a variation on that, which has alternating flaps of sandpaper and Scotchbrite. They are available in hardware stores and online. The ones I use are 3" dia x 1" wide x 180 grit; about $8. With a Combi-Flap Wheel in an electric drill, and a steady hand, you can "wear-in" the edges of a fingerboard in a few minutes. And not wear out your fingers.