After Dark RID said:

My advice is lots of layers, set your brush tool to a sheer density and plop 'em on, making sure the edges overlap some - I think dapples can often look unrealistic because people tend to paint them on like individual spots. Do this twice with different shades, light and dark, to increase blended look. Also, if you look at dapple greys IRL, you'll see the dapples look larger on their body and smaller on the legs and neck towards face. :)

Gem said:

I personally start using a very soft brush to make a nice dark blob, then I erase spots individually in somewhat of a lace work pattern. From there, I gently smudge them in the direction of the horse's fur. Set the layer to soft light then duplicate and adjust hue, saturation, brightness as needed