Just knit the stitch as usual, but instead of slipping it off of the left (if you're a right handed knitter) needle, insert the right needle (with the already knitted front loop on it) again into the same stitch still on the left needle through the back of the loop and knit as usual. Then you can slip the original stitch off of the left needle.
Hope this makes sense and helps!
Be Happy ~ Knit on !
