I wet the area first, applying water beyond the point that I knew I wanted the shadow to be. Then I used a number 10 round brush and applied a strong pigment into the water, along the very edge of the dog and the brown couch (or whatever it is they are sitting on).
The water softened the edges. I kept the board tipped so that the strongest color stayed at the dark shadowed edges of the dog’s bodies, and the water flowed into the pigment, rather that allowing the pigment to flow into the water.
Be sure that when you create the shadow areas under the dogs that the shadow falls on the opposite side of the light source. My light source is on the upper left, so my shadows all had to fall on the lower right.