I'm quoting BDL from the foodie forums below. Here's a pic of me following his directions with my 6" Sabatier Nogent and a strip of 120 grit sand paper. I followed up this with 800 grit to polish.
Rounding is a bit of a misnomer in my case as I really just took the sharp edge off until it felt comfortable in my pinch grip.
Padded with the knife in the cardboard sleeve it came in, wrapped in an old shirt sleeve, then between two pieces of box cardboard.
Took about 10 minutes.
1. Pad your table vise.
2. Tape your knife where it will be held by the vise.
3. Lock the knife in the vise, spine up, such that at least 1-1/2" of spine from the handle forward are clear of the vise.
4. Fold a piece of coarse sandpaper to about 1" wide (or, you may use a piece of sanding belt).
5. Use the sandpaper in a shoeshine motion on the spine, holding the ends of the sandpaper so they're at about a 30* from each side.
6. Keep working the paper until you have a rounded area as long and as rounded as you want.
7. Repeat with a medium grit paper so the spine feels very smooth to the touch.