To make the first "leg" of the V, slide the needle behind the V of the stitch above the stitch you are duplicating from right to left. Pull the yarn through.
You don't want to pull too tightly or the stitches will pucker, but if your duplicated stitches are too loose they will look strange as well. You want the stitch you are forming to actually duplicate the stitch below as much as possible. A gentle pressure is what is required.
