Plying can cause even the seasoned spinner trouble if they don't understand how plying works. The spin you create when you spin the singles will cause the yarn to ply back on itself. So traditionally we spin a Z twist single, spinning clockwise, and then ply with a S twist, spinning counter-clockwise. It is also important to note that it will benefit the spinner to be sure to keep an even twist on the singles. As you spin your singles, count how many treadles you are putting into the amount of yarn you spin before allowing it to take up into the bobbin. Be sure when you spin singles or ply that you bring your front hand back to the exact same place before allowing the yarn to take up. I ply and spin the yarn back to my tummy, counting the treadles, that way, the exact same length gets plied or spun each time. Be sure to always sit the same distance away from your wheel as well. This makes it easier to keep your twist even when spinning or plying.
A note here on plying off of balls. If you have trouble with extra yarn leftover, work on spinning more consistantly (unless you are intentionally making different sized yarns). This will reduce the waste, especially if you use a scale and spin each single from the exact same weight of fiber. You can also wind one bobbin onto a ball while counting the number of times you turn the handle. Then remove your new ball and divide your number of cranks in half. Now rewind this new number of turns onto a second ball, from the center of the first ball, this should be almost exactly half of one bobbin and therefore should have very little waste. You may want to rewind the original ball at this point, but it isn't neccesary.
Use a small sample with anything that contains silk before plying off of a center pull ball, as it can grab onto itself and make terrible snarls, that way if it will not work you haven't ruined an expensive fiber learning a lesson. Silk plies off of bobbins better than balls. Other very fine fibers such as alpaca may also be a little harder to ply off of balls, so if in doubt, test first.
Once you have your singles prepared, put your bobbin on the wheel, and unwind the leader, then thread it through the orifice. Now you need to treadle counterclockwise to take up the excess leader. Tie the ends of your singles to the leader with as small of a knot as you can, and treadle the same number of times as you did for the singles, but counterclockwise.
Now stop treadling and move your hand towards the orifice and see if your yarn is balanced. A balanced yarn will look just as the picture does. An over-twisted yarn will twist back upon itself to the left and an under-twisted plied yarn will twist back upon itself to the right. The action of winding onto the bobbin,as you ply, will remove a slight amount of twist, so you want to ply, just until it twists to the left a little. Count your treadles, and when you get the yarn to the point that it twists a small amount to the left, it should come out balanced on the skien. IF you bought some singles and re-spun them this will probably be about the same amount of treadles for all of it. If you have handspun singles, then you will have to check regularly because the twist is almost never consistent over the whole bobbin of singles, unless you are a seasoned expert, and even then there will be some variations, because its handspun, and nobody is perfect. Now bear in mind that after some time, you will develop a feel for how much twist is needed, and can achieve a balanced yarn without so much counting. Editors Note: The above was edited to desrcibe the action of winding on as removing a tiny amount of twist. Helen Grace-Muzzi was kind enough to clarify this point, so now we can all know exactly what is happening. Preiviously I was taught to ply to balance, but my skiens were always slightly underplied when I wound them off. Thank you Helen for the insight. Helen also added that knitting, crocheting and even skiening can also cause this needed twist to be removed, so play around and get just the right amount of twist for YOUR spinning style and project needs.
You will need to keep steady tension on the singles as you ply, otherwise they will ply back on themselves and make little nubs which are annoying when you didn't intend them to be there. While this can be an interesting intentional effect, it's best avoided unless you want a nubby yarn. You can snip the yarn and fix it, but an even tension will prevent the nubs in the first place.
It is important to keep the singles from getting tangled into one another, thereby making a huge mess that will make you want to throw the whole wad out into the front yard, and then maybe jump up and down on it for good measure. We don't want that to happen, so keep your singles separate and taut so that you can avoid knotted messes. Experience talks here and many people have a terrible time with plying, but once mastered it's the most fun. This is the step where you will get to see how your yarn will turn out, so it is very exciting once you rid yourself of the frustrating part.
It will help to hold the yarns so that they are kept separate, this way they do not become tangled. You can do this anyway you like, but here is a useful method of holding two singles, and will work with more singles as well but takes a bit of dexterity. I ply from two balls on my lap, but you can also use a Lazy Kate, I just find the balls easier to handle.
Because you so conveniently have five fingers on each hand, you can weave the yarns in and out of these fingers and ply as many as four, while keeping them tidy, snug and controllable. More than that, your guess is as good as mine, I wish you luck, but I don't know of anyone who does more than three plies, and I certainly don't spin finely enough to do more than three. I have moved my thumb in the picture so you can see the singles, but I close my thumb over them to keep them snugly contained.
This is basically all you need to know about plying as far as technique. The important thing to know, is how much twist to add to ply evenly, and this is done by recreating the number of twists per inch you put in the singles. Start there, and see where you are by checking the twist as described earlier.
As we close here is an example of a balanced skein and also an under-plied skein and an over-plied skein. I almost always under-ply, and as long as you are close to balanced you need not worry. Any of these skiens will balance with a nice hot soak in water, and air drying. This will relax the extra twist. Underplied yarn has excess Z twist, and over plied yarn has excess S twist.
One quick soak in a steaming hot tub (remember no agitation) will adjust the twist and redistribute it along the yarn to some degree. After soaking your skein, place it in the washer and spin out the excess water. Only spin, no water, no agitation or you will make a felt wad. Or you can wrap the skein in a bath towell and roll it up then step on it and sqeeze all the extra water out. After removing excess water, you can usually just hang the skein over the shower curtain. I don't use weights or block my yarn, because if you do this you can have more trouble with shrinkage later. Unless your yarn is WAY out of balance you shouldn't need to weight it while it hangs, but a bottle of spray cleaner is the ideal weight and has a handy hook for hanging on the ends. Place a wash cloth between the yarn and the spray trigger as a padding if you wish to block your yarn. As a final note, both of the above skeins after their soak were balanced perfectly and required no weight or blocking. Happy Spinning!