Also, lifelines will be easiest to place in a plain knit row, but can be placed in any row if needed.
What kind of string do you use for a lifeline?
Again, this is personal preference. And it's going to depend on what type of yarn you're using, what size needles, and the type of project. Scrap worsted weight yarn won't work well for a dainty lace project for instance, but may work just fine for a project made with bulky yarn. As a general rule, your lifelinen should be the same thickness as your yarn or thinner. You should also use a color that will show up against your working yarn. Don't use a black lifeline if you're knitting a navy blue project. Don't use white if you're knitting a cream project. You want to be able to see your lifelines easily. Trust me on this one. When you drop back down to your lifeline, you will have to pick up each stitch from the lifeline and get it back on your needle. The contrast will help immensely.
When in doubt? Dental floss! Waxed and unscented, though I personally haven't had any problems with mint-scented. White dental floss doesn't work all that well with paler projects, but for most things it works just great. Because it's nice and slick too, it will pull out of your work pretty easily. I used waxed, unscented dental floss for the photos in this post. As you can see from the top picture, generic works just fine!
So how do I put this lifeline in anyway?
There are 3 ways to put a lifeline into your work. The easiest way is to put one in the row you're currently working on.
First, take a length of string longer than your work. Thread it onto a tapestry needle. Run it through the active stitches on your needle from one side to the other.
If you have a stitch marker in your work, DO NOT run your lifeline through the stitch marker! Go around it. Then continue on the other side. If you go through the stitch marker, you'll anchor it in place and will have to either go back and pull out your lifeline or cut out your stitch markers. Not pretty. Don't do it.
This is how your lifeline and stitch markers should look. Notice that the lifeline is on the OUTSIDE of the stitch marker. I cannot emphasize this enough.
I didn't put in a lifeline and now I'm in trouble!
This brings me to the second way you can put in lifelines. You can put them in after-the-fact. This is a bit more time consuming that the first method, but it's doable.
This method takes awhile, but can be very helpful in a pinch.
What's the third way?
If you use interchangeable cable needles, they probably have a hole in the needle itself that you use to help you tighten the needle on the cable. You can thread this hole as if it's the tapestry needle. Then, knit your next row as usual and watch the lifeline almost magically insert itself into your row. When you're done with the row, unthread the needle and continue as usual.
A word of caution though: if you're using closed ring stitch markers, your lifeline will also go through them! As we've already discussed, this is bad. Options? Remove your SMs while working that row or use removeable stitch markers.
So what do I do when I make a mistake?
Well, first, try other methods to fix your mistake if possible. Ripping out your work should always be a last resort.
If you've exhausted all other possibilities, then take your needle(s) out of your work, and rip out your work to the row just above the lifeline. Then, with your knitting needle ready, slowly pull out this last row as well, catching the row with the lifeline as you go. Take your knitting needle and slowly pick up these stitches one by one, putting them back on the needle. For now, leave your lifeline in place.
Go all the way across. If possible, make sure the stitches are positioned correctly, with the right leg of the stitch to the front. Count your stitches to make sure you have them all.
Once you're sure that everything is positioned correctly and that you have all of your stitches, you can take out your lifeline. BUT you probably shouldn't! Why? Because you might need it again? If you goofed once, you may goof again. Leave the lifeline where it is, just in case.
When can I take out my lifelines?
Even if you have several lifelines in your project, leave the earlier ones in just in case you find an error below the newer lifelines.
When your project is completely done, you can safely remove your lifelines and block your project.