I'm sure that for a lot of you the method to duplicate stitching is about as obvious as it gets, but I decided to do a short tutorial so if any of my granddaughters decides she wants to try a new handcraft she can refer to this.
First, find the right kind of knitting to use as a base for your duplicate stitching. The knit stitching has to be large enough to be able to stitch over it (the little teeny stitches in many machine-stitched pieces won't work for this). A bulky sweater is usually good, or, as with the example in the photos, a machine-knit piece that uses a lacy pattern. Or, of course, something you knit yourself that you want to do a bit of decorating on.
Take your knit piece to a store that sells embroidery thread and decide what color(s) of thread you want to use. Embroidery thread is cheap and it goes a long way, so you can get several colors that you like and make a final choice when you start the project. Also, if you don't have any, buy a packet of needles for embroidery. They're bigger than sewing needles, with bigger eyes to get the thread through, and also less sharp. Regular sewing needles won't work for embroidery.
To work with the embroidery thread, first find an end (without taking off the little paper thingies that hold the skein in order) and pull out a length. I usually pull out a length that goes from one of my hands to the other when I've stretched my arms all the way out. Anything beyond that is more difficult to work with. The thread won't get tangled doing this (well, hardly ever) - in fact, embroidery thread rarely has any problems with tangling which is a big bonus compared to working with yard skeins!
Embroidery thread is actually made up of 6 smaller threads. It's generally best to use only two or three of these smaller threads at a time in your work - otherwise it's too bulky. To split the embroidery into smaller parts, take one end and untwist the threads.
Take 2 or 3 of them - they don't have to be right next to each other - and start separating them from the other threads. Pull them all the way apart.
Take the segment of 2 or 3 mini-threads that you're going to work with and make a nice BIG knot at the end. The knot should be extra-big because otherwise it will pull right through the knitting stitches. As you can see, I have NOT doubled up the thread - one end is left free.
Decide where you want to start your duplicate stitching and insert your needle from below (the wrong side of the knit fabric) and anchor the knot. For your first stitch, insert your needle from left to right through the middle of the stitch ABOVE the first knit stitch that you want to sew over, and pull the thread through.
Then insert the needle through the TOP of the stitch below (which is hidden under the stitch you're working on). I hope the photo is clear enough to show you what I mean - couldn't get a closer one that was sharp. Pull the yarn through - you've done a full stitch!
If you mess up and want to take out one or more stitches (or the whole section), simply slide the needle off the thread and use it to pull the thread out in the reverse order that you sewed it in.
That's it! Easy peasy, and SO fun to add color and decoration to existing pieces!