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 a magical join method that utilizes felting to blend old yarn into new. Note: The yarn must be an animal fiber capable of felting for this to work. Here, the method is shown with two different colored yarns, as if joining to prepare for colorwork in the midst of a knitted piece. Make sure you leave at least 5 inches of the previous ball available before you begin felting in the new ball. You will need the slack when performing the felting.
First, separate the plies of one end of the yarn to be joined into two separate groups.
Unravel about 2 inches worth and snip away half of the strands. Do the same for the end of the other ball of yarn to be joined with this one.
Wet the yarn ends (using water or saliva) and fold over an inch of each yarn end, hooking them around one another. Lay these in the center of your palm.
Create agitation between your palms or against your pant leg. Roll vigorously back and forth beneath your palm until yarn begins to felt to itself at each hook.
You will be left with a fine looking, neat and seamless join.