I'm sure you can use wet/dry sandpaper in successively finer grades. A sharpening block used for knives is also useful. Just make sure you keep the pen in the same plane and direction each time you pass through the grit. IE don't rotate your pen as you sand, otherwise you'll get a round bottom. Also, a 20x loupe is necessary. The final passes should be to bevel or round off the sharp edges. I used Mottishaw's youtube videos as a guide for my attempts. You will need to sand, test, sand, test, and resand and retest the nib before you get it just right for your hand. I think what is good for my hand probably won't work for someone else's hand as I write in a very particular style and pen slant. It is even more noticeable in a cursive italic grind over a stub grind. My factory stub on my TWSBI has very rounded corners and is thicker, hence able to accommodate many hand and pen positions on the paper.

As I only had 400x wet dry silicone carbide paper in my tool cabinet, I used that. I ended up finishing it off on the bottom of one of my ceramic plates, where there is no polish When I go to the hardware store, I'll pick up some finer paper. My pens still need a final polish to them. There is still a bit of road feel to them, especially so when I hold the pen to paper slightly differently. The sweet spot is very narrow.