I tried to do it by hand before and it was tedious work. You can certainly do it but I found that soon as I got into it I wished I never started. since I don't have three hands, I found this to be a quick way to ruin my satisfaction with a nice rod. clamping in a vise and then using strips of sand paper with both hands to sand around the handle - I just found it miserable. I've seen example of some nicely reshaped handles but I'll never do this again.
Since then I've done this a couple times on a metal lathe with a through-hole headstock. Wrap the blank with paper towel and carefully clamp the shaft in the three-jaw chuck. It would be so easy to crush the shaft so finesse is critical. Then it's an easy process to sand and true the entire handle to that clamping position. Clamp as close to the handle as possible to reduce wobble. On long handles - drift rods and spey, I'll turn a centerbit into the butt end so I can use the tailstock for support. Nice to have a less wobbly handle to sand. Then it's a simple plug repair afterwards - provided it didn't have a decorative metal butt plug/cap. Moderately slow speed and light sanding pressure does nice work.
I've cleaned up soiled and slippery handles like this as well. Fortunately I have access to a suitable lathe.