with your needle inserted in the next stitch, lay the new yarn over the needle as shown - tail to the right, ball of yarn over the needle to the left

knit the stitch (with whatever yarn you are using)

take the main yarn back over the needle -- it has been caught in the previous stitch

with the new yarn held down, knit the next stitch

the new yarn is now caught and locked down by this second stitch - repeat this for approx. 10 stitches -- to weave a new yarn in, or an old yarn out