Scrap amounts, or a whole skein: any reasonable amount of yarn can be hand-wound into neat and tidy center-pull balls. There's a video, followed by the non-video illustrated version.
Video
Here is the non-video illustrated explanation.
Begin by laying the yarn tail between the first two fingers of your non-dominant hand (left hand in the illustration). This tail will become the center-pull.
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| Lay the tail between your first two fingers. This will become the center-pull |
Now trap down the tail with your thumb, then wrap the yarn over your fingers 10 times to form a (very) mini hank.
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| Mini-hank wound over first two fingers |
Once you have ten loops over your fingers, slip this mini-hank off your fingers, pinch it up and wind more yarn over its middle, meaning at 90 degrees to the direction of the original wind.
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| Wind over the middle |
For a small ball, the last two steps are these
-- As you come to the last wrap, lay a forefinger alongside the winding, then wind the very last wrap over it, as shown below. Then, pull up tightly.
-- Insert the tail-end UNDER the last wrap, in the direction from your forefinger towards the palm of you hand, meaning inserting towards the center-pull: red arrow in illustration above. If you've done this last part correctly, the entire scrap of yarn will unwind from the center pull without a knot in its tail. Inserted the other way (from palm towards finger) will leave a small granny-knot in the tail. Give a final gentle tug to seat the tail and there's your tiny center-pull ball. This is a very convenient, very quick way of organizing scraps.
For the larger ball version, keep winding over the center mini-hank maybe 20 or 25 times, until you have a good mass into which you can sink a "claw" consisting of your thumb and middle finger, as shown.
In the upcoming action, each of the fingers on your non-dominant hand gets a different role.
--The claw of thumb and middle-finger is the axis around which the ball will rotate. Where these fingers are, the top and bottom ball-eyes will form.
--The forefinger provides rotation by a ratcheting action, meaning it will rotate the ball towards the palm (red arrow) one small step after each wrap: "to ratchet" means to "proceed by steps or degrees."
--The last two fingers hold down the center-pull against the palm so it does not get lost.
The action is this: the dominant hand winds the yarn over the top of the ball, first bringing the yarn towards you, then away over the top of the ball, around the back, and so up from the bottom again. In the illustration below, the yarn has been passed over the top of the ball, is now around the back, and will shortly be drawn forward again, up from the bottom, and so over the thumb. In this way, every wrap slightly overlaps the thumb.
As the ball is rotated by the forefinger, the yarn slides off the thumbnail in the direction of the green arrow. If each rotation is equal, the yarn slides off at even intervals: this is how the very pretty and regular pattern of yarn is laid down. The yarn is also wrapped slightly overlapping the middle finger as it passes around the back of the ball, and the same slide-off occurs at each ratchet-step.
As to just HOW the forefinger provides rotation, in the illustration below A and B are both your forefinger, just in different places. When the forefinger is in the A position, it is just about to plunge into the ball. As soon as the wrap goes by, the forefinger does plunge into the ball. Then, still plunged, it is drawn towards the palm, into the B position (solid blue arrow). This make the ball rotate one small step around the "claw" axis of your thumb and middle finger. In the illustration, the forefinger in its B position has just completed the rotation and is being lifted out of the ball.
After each little rotation, the forefinger is raised and put back into the A position (blue d-o-t-t-e-d arrow), waiting above the ball for another wrap to go by. With each wrap, the forefinger again plunges into the ball and draws the ball towards the palm into the B position. It is in this sense that the forefinger is "ratcheting" the ball. In other words, it is the repeated sinking, drawing and lifting action which rotates the ball by a small degree each wrap, and so each trip of the forefinger from A to B is a "ratchet step."
If you find this confusing, then, even if you HATE videos, consider watching just the "wrap-and-ratchet" action of the video at the top of this post. That specific action starts at 2:37 (2 minutes and 37 seconds) into the video.If you are wrapping and ratcheting with your thumb and middle finger "claw" always in the same place, deep eyes would form at each finger, and the ball would start to become egg-shaped. The steeper the egg, the more yarn would try to fall off the sides. So, when you start approaching egg shape, you have to stop wrapping and change position. Simply sink your thumb and middle finger "claw" into the ball in a new place, and then start wrapping and ratcheting in this new position.
You may wonder what happens to the center-pull when you wind over it, and the answer is, nothing happens. As long as you give the center pull yarn a tug every so often to keep it at the correct length, and as long as you don't lose track of it, the center-pull yarn will travel along the inside the ball from the old eye to the new eye, and appear there. When you first go to tug on a center-pull ball which has been wound using many changes of direction, the center pull may be stubborn. Have faith! Insist! Keep tugging and the stubbornness will subside. After these first few tight tugs, the center-pull will have created a straighter path from the heart of the ball where it originates, to pull more easily outward.
Three last thoughts.
First, just because the ball is capable of being center-pulled does not mean you have to use it that way. In fact, center-pulling may introduce unwanted twist. For more info, there's a whole TECHknitting post about that, but the short version is, you may want to unwind from the outside of the ball to prevent biasing. If you for sure are going to unwind from the outside, you can wind the ball just as shown here, but without keeping track of the center-pull.
Outside-unwinding is conveniently done either via a yarn lazy-suzan* (either commercial or home-made) or a yarn bowl or similar container. If outside-unwinding a center-pull ball, tuck the center-pull into the eye of the ball, having first knotted-on a scrap of contrast color yarn so you can find it again.
And finally, we are winding yarn here, not the innards of a baseball. Keep the tension of winding as loose as you can: tight enough so the yarn doesn't fall off in every direction, but no tighter. Tight winding stretches the yarn now and leads to problems later--uneven gauge, garments which shrink mysteriously the first time they are wetted and even yarn which offers to come apart. Loose is the watchword when winding yarn into balls!
--TK
* Geek note: if you know ahead of time you'll be using a lazy-susan with a spike, you'll have to re-mount the ball at every place you changed direction of winding, because the placement of the eyes has changed. Therefore, you may choose to change direction of winding less often than if you were unwinding via yarn-bowl.
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Related posts about yarn handling
Yarn organization for color knitting
Quickly unkinking yarn with a steam iron (video)
Winding a skein into a ball of yarn
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