If your project requires more than ball of yarn, you'll need to join them together to ensure your work flows seamlessly. Learn how to do that below.
Take the end of the yarn you're knitting with and the start of the new ball of yarn. You'll need at least 10cm.
Carefully split the yarn in two on both pieces as shown above.
Gently pull away one half of the yarn on each piece.
Lay the two strands of yarn on top of eachother, so that they overlap as shown above.
Roll this section of yarn between your fingers to merge the fibres a little. You can now continue knitting your project.