It's easy!
Same as a regular balsa nose cone but instead of a wood dowel glued into a big block of wood you glue a round toothpick into a small scrap of balsa.
Chuck it into a Dremel tool, turn it on and hold it against some sandpaper.
If ya screw it up...throw it away and grab another toothpick and another scrap of balsa.
Eventually you'll get it. Besides that you can get rid of all the balsa scraps you have been saving.
Or carve the sholder to fit the body tube and just sand it round by hand...Gee! it's only 1/4" in diameter!
It's easier than you think.
sandman