Insert your right needle into the stitch and knit it, as usual, BUT don't slip the original stitch off the left needle yet. Bring the yarn to the front and purl the SAME stitch, still not slipping the original stitch off the left needle. Bring the yarn back again and knit the SAME stitch once more, this time slipping the stitch off the left needle. So you have essentially made three stitches where there originally was just one.
Hope this helps.