Horizontal: Bring threaded needle out from back to front at the base of the V of the knitted stitch you want to cover. *Working right to left, pass needle in and out under the stitch in the row above it and back into the base of the same stitch. Bring needle back out at the base of the V of the next stitch to the left. Repeat from *.
Vertical: Beginning at lowest point, work as for horizontal duplicate stitch, ending by bringing the needle back out at the base of the stitch directly above the stitch just worked.