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 the easiest way to join a new ball of yarn in the same color. It also conveniently secures the yarn ends so you can avoid darning them later on. You can knit in the new ball of yarn anywhere, but it is best to place it somewhere that is less visible, like under the arm or by a seam so that the stitches worked with two strands will not stand out.
Hold the remainder of the old ball of yarn together with the beginning of the new ball of yarn. Wrap these together around your hand as you would a single strand of yarn you were going to knit with.
Knit the next three stitches with both strands held together.
Drop the strand from the old ball of yarn and keep knitting on with the new ball.
When you come to these 3 stitches in your next row or round, be sure that you work the double strands together as one stitch.
Later on, go back and trim the yarn ends hanging on the wrong side of the work. Leave about an inch of tail so they can’t work themselves loose.