Amanda sent me in the question, "Do you set the twist in singles, then ply them together? Do you set in every single, ply, then set the ply's twist? Or do you spin your yarn, ply it and the set the whole thing at once?"
When plying, you set the twist after you've plied. There's no need to set the twist in the singles.
That said, the twist begins to set the moment the newly spun yarn is wound onto your spindle or taken up onto the bobbin of your spinning wheel. Even 5 minutes rest will begin to set the twist. So if your aim is a balanced yarn, I'd recommend saving a newly spun length of yarn, folded back on itself for the ply you want -- 2, 3, 4, or whatever ply. Mostly I do 2 ply or 3 ply myself.
That freshly spun, self-plied length is a great sample to refer back to when you're plying, to make sure you put enough twist in when plying. That way, when you wash the plied skein to set the twist, the warm water will relax the fibers and you'll find you have a balanced skein when you're done.
Now, if you've already washed your singles and decide to ply later, I wrote about that in my blog, you can read about handling that in the entry, How do I ply set singles?
And I have another entry on making a good looking two-ply yarn.
The yarns in the photo are from the local guild fiber exchange -- we all exchanged 2 oz of white fiber (romney cross singles, corriedale top 2-ply, medium wool singles, merino top 3-ply, merino/tencel roving 3-ply) to overdye, spin, and make something. Well, I make yarn ;-) so far, anyway. There are plans -- the two medium wools are singles, to be crocheted into a little tote; the corriedale top will be a Knitty headscarf, the merino will be socks with white lace cuffs on them, and the merino/tencel will be a sock machine pillow top. Yes, really! the Victorian SCM manual that's available on sockknittingmachinefriends has a great knitting pattern for a "bath matt" but I'm thinking it's more the size, I expect, for a small pillow top.
Oh yes -- in the realm of curious personal trivia, my name was Amanda for a day -- then my mother heard the nurses in the hallway say "Amelia", and decided to name me that instead. I have wondered how different it would have been, to have been named Amanda. And if you click on the picture to see the skeins more closely, that's me and my sister in the PJ's in the photo in the corner as little 'uns. Easier to explain than to edit the picture down to just the skeins, LOL.
Thanks for asking, Amanda! if you or any of the other readers want more info on any of this, feel free to comment or email me.