The way I learned to join another yarn to my crochet project is to add the new yarn to the stitch you are doing, when there are two (2) loops of the stitch left on your hook.
For example, I'm going to add new yarn to a double crochet project. So I would do my dc (double crochet) stitch by beginning with:
I just bought 3 skeins of blue yarn to crochet a sweater for my granddaughter.
Another name for a hank of yarn would be a skein of yarn.
skein
To skein yarn without a niddy noddy, you can use a chair or your arm to wrap the yarn around in a loop, creating a skein.
Yarn comes in different "gatherings"--ways that it can be sold. Your question mentioned 2 out of the three: ball of yarn and skein of yarn. The third would be a Hank of yarn. A hank is a skein of yarn (which is yarn in a large oblong shape, and tied in a few places around the skein to maintain a bit of control over the yarn staying in that loose shape), which has been twisted around itself to make for a smaller and more controllable yarn, which is easier to pack, and store. A hank also is a bit more manageable than a skein of yarn. A ball of yarn would be wound from a skein or a hank.
The yarn that is attached to the skein is often referred to as the "tail" or "end" of the skein. This tail is used to pull off the yarn for knitting or crocheting without unraveling the entire skein. In some cases, it may also be called the "working yarn" once it is being used in a project.
A coil of yarn may be called a hank or a skein. Yarn is typically sold in a ball or a skein. A hank is different in that it is a big loop of yarn that is twisted up for easy shipping.
A small amount of wool yarn is typically referred to as a hank rather than a skein. A hank is a looped bundle of yarn that needs to be wound into a ball before use. A skein is a different form of yarn packaging and usually contains a larger quantity of yarn.
While there are general standards, there are no absolute standards for the amount of yarn in either a skein or a hank. The amount depends on the weight of the yarn and which fibres are including in its spinning.
look at the pattern...... it usually says how much..... i.e.: 5 skeins of yarn As noted above, the pattern generally states how much yarn to use for that pattern. If you are trying to design your own pattern, however, then the process is more complicated. There is no general rule for how much yarn in a pattern, even as to type of pattern, because each is so different. Different scarf patterns, for example, would take different amounts of yarn. Generally speaking, I can probably make a scarf if I have 7 to 10 ounces of yarn. If I am making a hat for a child, I can do it with one 3.5 ounce skein with yarn leftover. For an adult, it depends on the pattern. Sometimes one skein would be enough, sometimes two would be needed. Afghans require more yarn, but amount varies with yarn, hook, and the size of the afghan as well as the pattern. One can take a baby afghan pattern and use chunky yarn to give a larger afghan, but amount of yarn required will be different. If you have a pattern and it gives you the yarn amount in skeins but not ounces, and you want to use a different yarn, then you need to find out how many ounces in that type of yarn, then buy the required ounces in a yarn in the same weight category.
A flock or a skein (as in a skein of yarn) (pronounced skayn)
The length of yarn in a skein varies greatly with brand, variety, etc. You can determine the length of the yarn by looking on the front of the label, where it is listed along with weight, recommended needle/hook size and dye lot