When you're ready to put stitches on hold, take one of the open-ended needles and one of the lengths of leather cord. Hold the very end of the leather cord in one hand, while twisting the needle onto it with the other hand. Once the needle is securely on the cord, you can thread your stitches from your knitting needles, onto the stitch holder cord. Once your stitches are on hold, pull the needle off the leather cord, and tie a simple knot with the two ends of the leather cord to keep everything nice and secure.
Then, to take the stitch holder out and start knitting again, simply put the stitches back onto your knitting needles, untie the knot in the leather cord, and pull the cord out of your stitches.