To begin, slip the first stitch on your left needle to the right by inserting the tip of the right needle into the front of the stitch. Repeat for the second stitch on the left needle.  

Now you have two twisted stitches on the right needle; they are facing the opposite direction from all of the other stitches. Slip them back to the left needle so that they remain twisted.

Now knit the twisted stitches together through the back loop, inserting the point of the right needle behind the back needle and into both stitches at once. Try to keep the stitches as loose as possible.

You have decreased one stitch and created a stitch that slants to the left.