Matt,
This is one of those things. It's hard to describe but I'll try. Not that it's so hard to actually do, but describing it is.
First, I used a sure nuff tomahawk drift that I purchased. I could have made one but mine might have been shaped a little different and that would mean any handles I bought would have to have some customizing. So I bought one to get the shape in mind and have a gauge to go by for the size as well as the taper. The taper is important. (See footnote) I made this one because I had the material and wanted to get a good understanding of it. So, number one, have a target size and shape.
Second, Decide the size on the other end, assuming it is close to the shape of the top, just smaller. Then I try to make everything in between taper gently to both sizes. A spoke shave is nice for straight grained woods, but it wanted to tear this curly Maple. Make it a couple inches too long and trim it after you get the head settled into a place you like that allows the handle length you want. Allow for the finish sanding to get it's final resting place.
Footnote: I mentioned getting an understanding. This is what I mean. If the taper is too shallow, it might conceivably slip too far up the handle as you use it. If it's too steep, the head wont grip the handle and it will always get loose. Think of the taper on a drill chuck. Buying a drift, even a cheap one, will get your eye calibrated to an approximate size and taper.