As for many knitted objects, 'finishing' is the final step in creating handspun; finishing sets the twist and gives the yarn a finished look and feel. Since I wrote about the blocking process for handknits a while ago, I thought I should follow up with the process for finishing handspun.
This photo shows a recently handspun skein just after it came off of my spindle. You can tell that my yarn is not very smooth and still wants to twist a bit. This most likely results from residual twist in the yarn, which can be unevenly distributed throughout the skein
The first step in finishing handspun is the same as for finishing handknits: soaking. I fill up my bathroom sink with warm water and a generous squirt of Soak. I immerse my yarn in the water, being careful not to agitate it and thus felt it, and let everything sit for half an hour.
The next step is my favorite part of the cleaning process, which is unfortunately not pictured: thwacking. I do this either by holding one end of the skein and throwing the rest against the side of the tub, or by grabbing the skein at both ends and pulling tightly. Thwacking helps redistribute twist and even out the yarn, so I do it several times while holding the yarn in different places. Then I hang the skein over the tub, unweighted, to air dry.
That is the whole process! You can see from the finished result that the yarn is much smoother than it was at the beginning of this process. As long as you've done a fair job of balancing the twist during plying, this finishing process should smooth out any residual twist in your handspun.