Ok, I think there is a video about it on this very site.
You knit into the stitch, as you'd normally do, but you don't slip off the stitch off the left hand needle.
Then you insert the needle into the back of the stitch (hold your right hand needle under the left hand needle and insert the tip into the stitch by going into the stitch and going backwards with the right hand needle)
Then, wrap yarn around needle and pull through. Then drop the initially knitted stitch off the left hand needle.
Honestly, it's easier when you see it, and if I am not mistaken, there is a video on here, so just go to the video section and look for kfb