A reverse-tapered handle really helps keep your hands from cramping up as you row. Before making the handles, try to get the shaft more or less round. I make the shafts from a 2x4 ripped into 1.5 x1.5 sticks. Then I set the tablesaw to 45 degrees and knock the corners off, making them octagons. For the handles, I start by cutting a groove where I want the handle to stop - this helps sets the depth and ensures uniformity. Next, I take a chisel and shave the handle to sort of roundish, tapering it to the groove. I finish up with a belt sander with very coarse paper and make the handle as round as possible.