There are many different ways to join a new length of yarn to the one that is already being used. Like most knitting techniques, the method you choose depends on your personal preference and the specific project situation. Experiment with each and pick what works best for you!
This is an easy join and works well on those yarns that cannot be felted. It requires a yarn needle. It can make things easier if you use a small needle and break the yarn end instead of cutting it. When you break the yarn, the tapered end it creates allows the tail to pass more smoothly through the shaft of the yarn.
First, thread one of your yarn ends on a yarn needle.
Insert the needle back into the strand of yarn and draw it through about two inches worth (depending on how slippery the yarn is you can increase or decrease this measurement), making sure to keep it inside the wound plies.
Pull the needle and yarn tail out along the strand of yarn, leaving a loop at the end.
Thread the other piece of yarn on the needle and pass it through the loop you created with the first length of yarn.
Run this yarn back along itself as you did with the first.
Tug on the yarn ends to tighten the loops.
Trim the remaining excess yarn.