Life lines are something the you most commonly hear mentioned when talking about lace knitting but I would use one in any knitting project where I wanted to protect my work so far.
You can put a life line anywhere in your knitting by threading a blunt tapestry needle with sewing cotton and threading it through every stitch in a row.
You don't need to take your stitches off the needle to do this but be careful not to thread your cotton through any stitch markers.
Once your thread is through all the stitches just ignore your lifeline and work on with your knitting.
If you get to the end of your work without needing to go back you can just pull the cotton thread out. But if you do make a mistake above the life line you will not be able rip back beyond your life line so the work you are happy with is protected.
This is why I am such a fan and recommend using a life line anywhere you are happy with your work so far and are about to start a new stitch pattern, colourwork section, shaping or anything else where you think you could go wrong.
I guess I've just added a reel of sewing cotton to your knitting bag essentials.