I have just posted this on Flickr.com, it is one of my favorites. It is a demo painting that I do, I have always had fun with it. First, you place the torn masking tape down for the fence, then wet the paper with a big soft brush. Then mix a wash in the lightest color you will use in a small dose cup and pour it over the paper, leaving some white where the sunlight will be coming from. After the shine disappears sprinkle with salt and let dry. After it is dry brush off the salt and rewet the paper gently, using only one stroke at a time so you don't disturb the underpainting. Mix another wash in a dose cup of the second lightest color. Repeat these steps until you have done the middle darkest color.
After it is dry and you have brushed off the salt, carefully peel off the masking tape and paint shadows and details using the colors you used in the washes and greys made from contrasting versions of those colors. I used yellows so my greys and darks were made from mixing violet and yellow together. You will love the opportunities to play with this technique, do a lot of these. Enjoy.