1.
Slip 3 stitches purlwise, or slip as many stitches as you need to slip to reach where your working yarn is currently coming from.
2.
Place the tail of your new strand of yarn between your needles, so the tail is at the front of your work and the working yarn is at the back alongside the previous working yarn.
3.
Slip the stitches on your right needle back onto your left needle.
4.
You can now begin knitting with your new strand of yarn.