When gluing up the handles, I get epoxy all over my hand. I stick them in a drum of 100 grit silicon carbide blasting media. Then, after the glue cures on the handle ( and my hands), I use my grit covered fingers to sand in the finger choils
When I don't have time for that, I use small contact wheels in diameters from 3/4" to 2" and half round or round files/rasps. Wooden dowels in various diameters can be wrapped with sandpaper to sand these areas smooth and give them a gentle curve. Avoid any pointed projections. I personally dislike most handles with multiple finger grooves. A smooth palm swell coming out of a forward finger choil, and ending in a slight "bird's head" butt, is what feels and works best.