Once the blade and handle are fitting together satisfactorily, finish cutting out the handle. Then, you can start on the artistic part of the project. The temptation may be to chuck a round over bit in a router, a quick swipe of abrasives and call it done. If you're happy, that's fine. However, the nicest handles don't just have rounded corners, they have an oval cross-section, and I believe it's worth the extra effort. Half-round rasps and files are the tools for this task. To get a consistent shape, pencil on a few guidelines and rasp a series of facets rather than trying for the complete curve in one shot. There are about four stages to each corner, so the complete circumference actually gets worked at 16 angles. Sounds like a lot, but it goes quickly and seems to be the easiest way to get the desired shape without losing track of the curve. Blend the different angles into one curve and follow up with files and abrasives until you have the level of finish you want. Use your hands as well as your eyes to ensure that you're getting the shape correct; if something feels wrong, even if it means going back a stage, do it now because it'll yell at you when the saw is done.