Set the twist in your singles as you would for a plied yarn:
Wind the yarn around a niddy-noddy (or your arm, or the back of a chair -- whatever's handy) and put 4 figure-8 ties evenly spaced around it; more if you are worried about it getting messed up, such as with a very fine, high-twist yarn. (A figure 8 tie is just that -- make a figure eight with your tie-yarn around and through the width of the spun yarn -- effectively this makes it into two joined bundles at the figure 8.) Remove the skein from the niddy, arm, or chair.
Fill a sink with warm water and a little wool wash (or dish liquid or shampoo). Put the skein in -- don't agitate. Let it sit 20 minutes.
Take out the skein, squeeze the soapy water out of it, empty the sink. Refill the sink with the same temp water and _no_ soap. Put the skein in that for 20 minutes (to rinse-soak). If needed, repeat the rinse.
I roll my skeins in a towel when I'm all done and squeeze them well, to get out as much water as I can. There's also "wuzzing" ... where you whir the skein around in a circle while holding tight to it.
Then hang the skein to dry. If you don't want it to curl (for singles), you might need to weight it -- I usually just put a handtowel through the lower part unless it's very high twist, then I resort to soup cans.
Whew. All that said, I did that with a very high twist single recently and still found it was twisty. So I steamed it -- in the steam from a boiling tea kettle -- it still didn't settle down completely, so now I have it wound tight onto a niddy noddy to set the twist that way. May take a month or so, but it should calm down eventually. Since it's part of a year-long study group, I don't mind waiting!
(posted by me this day on spindlers)