When you're happy with the shape, chamfer the end that
will rest in your palm and use abrasives to lightly knock
back all the edges until it feels comfortable.
Finally, all that’s left is fitting the tool. For tools where a
perfectly straight fit isn't essential, bore the required
hole after finishing the handle. For more critical tools
like chisels, fit the tool to the blank first and then
shape it to the orientation of the blade. This also gives
you an easy means to hold the more tapered blanks while
you work on them. Simply clamp the blade in the vise and
you’ll have unimpeded access to the handle.
Finish with oil, wax, varnish – whatever you desire. And
if you're bemoaning the fact that you don't even have
a block plane, use your imagination. I've shaped a rasp handle entirely on an inverted belt sander.